Chapter 1. Introduction
Chapter 2. Organic matter decomposition and the soil food web
Chapter 3. Natural factors influencing the amount of organic matter
Chapter 5. Creating drought-resistant soil
Chapter 6. Key factors in sustained food production
Chapter 7. The role of conservation agriculture in organic matter deposition and carbon sequestration
Chapter 8. Conclusions
Chapter 1. Introduction
http://www.fao.org/docrep/009/a0100e/a0100e04.htm#TopOfPage
On the basis of organic matter content, soils are characterized as mineral or organic. Mineral soils form most of the world’s cultivated land and may contain from a trace to 30 percent organic matter. Organic soils are naturally rich in organic matter principally for climatic reasons. Although they contain more than 30 percent organic matter, it is precisely for this reason that they are not vital cropping soils.
This soils bulletin concentrates on the organic matter dynamics of cropping soils. In brief, it discusses circumstances that deplete organic matter and the negative outcomes of this. The bulletin then moves on to more proactive solutions. It reviews a “basket” of practices in order to show how they can increase organic matter content and discusses the land and cropping benefits that then accrue.
Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process (Plate 1). At any given time, it consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus (Figure 1).
Plate 1 Crop residues added to the soil are decomposed by soil macrofauna and micro-organisms, increasing the organic matter content of the soil.
A.J. BOT
| FIGURE 1 Components of soil organic matter and their functions |
Most soil organic matter originates from plant tissue. Plant residues contain 60-90 percent moisture. The remaining dry matter consists of carbon (C), oxygen, hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Although present in small amounts, these nutrients are very important from the viewpoint of soil fertility management.
Soil organic matter consists of a variety of components. These include, in varying proportions and many intermediate stages, an active organic fraction including microorganisms (10-40 percent), and resistant or stable organic matter (40-60 percent), also referred to as humus.
Forms and classification of soil organic matter have been described by Tate (1987) and Theng (1987). For practical purposes, organic matter may be divided into aboveground and belowground fractions. Aboveground organic matter comprises plant residues and animal residues; belowground organic matter consists of living soil fauna and microflora, partially decomposed plant and animal residues, and humic substances. The C:N ratio is also used to indicate the type of material and ease of decomposition; hard woody materials with a high C:N ratio being more resilient than soft leafy materials with a low C:N ratio.
Although soil organic matter can be partitioned conveniently into different fractions, these do not represent static end products. Instead, the amounts present reflect a dynamic equilibrium. The total amount and partitioning of organic matter in the soil is influenced by soil properties and by the quantity of annual inputs of plant and animal residues to the ecosystem. For example, in a given soil ecosystem, the rate of decomposition and accumulation of soil organic matter is determined by such soil properties as texture, pH, temperature, moisture, aeration, clay mineralogy and soil biological activities. A complication is that soil organic matter in turn influences or modifies many of these same soil properties.
Organic matter existing on the soil surface as raw plant residues helps protect the soil from the effect of rainfall, wind and sun. Removal, incorporation or burning of residues exposes the soil to negative climatic impacts, and removal or burning deprives the soil organisms of their primary energy source.
Organic matter within the soil serves several functions. From a practical agricultural standpoint, it is important for two main reasons: (i) as a “revolving nutrient fund”; and (ii) as an agent to improve soil structure, maintain tilth and minimize erosion.
As a revolving nutrient fund, organic matter serves two main functions:
- As soil organic matter is derived mainly from plant residues, it contains all of the essential plant nutrients. Therefore, accumulated organic matter is a storehouse of plant nutrients.
- The stable organic fraction (humus) adsorbs and holds nutrients in a plant-available form.
Organic matter releases nutrients in a plant-available form upon decomposition. In order to maintain this nutrient cycling system, the rate of organic matter addition from crop residues, manure and any other sources must equal the rate of decomposition, and take into account the rate of uptake by plants and losses by leaching and erosion.
Where the rate of addition is less than the rate of decomposition, soil organic matter declines. Conversely, where the rate of addition is higher than the rate of decomposition, soil organic matter increases. The term steady state describes a condition where the rate of addition is equal to the rate of decomposition.
In terms of improving soil structure, the active and some of the resistant soil organic components, together with micro-organisms (especially fungi), are involved in binding soil particles into larger aggregates. Aggregation is important for good soil structure, aeration, water infiltration and resistance to erosion and crusting.
Traditionally, soil aggregation has been linked with either total C (Matson et al., 1997) or organic C levels (Dalal and Mayer, 1986a, 1986b). More recently, techniques have developed to fractionate C on the basis of lability (ease of oxidation), recognizing that these subpools of C may have greater effect on soil physical stability and be more sensitive indicators than total C values of carbon dynamics in agricultural systems (Lefroy, Blair and Strong, 1993; Blair, Lefroy and Lisle, 1995; Blair and Crocker, 2000). The labile carbon fraction has been shown to be an indicator of key soil chemical and physical properties. For example, this fraction has been shown to be the primary factor controlling aggregate breakdown in Ferrosols (non-cracking red clays), measured by the percentage of aggregates measuring less than 0.125 mm in the surface crust after simulated rain in the laboratory (Bell et al., 1998, 1999).
The resistant or stable fraction of soil organic matter contributes mainly to nutrient holding capacity (cation exchange capacity [CEC]) and soil colour. This fraction of organic matter decomposes very slowly. Therefore, it has less influence on soil fertility than the active organic fraction.
Chapters 2 and 3 deal with the transformation of organic matter by soil organisms and with natural factors influencing the level of organic matter content in the soil. Chapter 4 discusses the various management practices that affect the accumulation of organic matter in the soil. Chapter 5 examines how to create drought-resistant soil, while Chapter 6 explores various aspect of sustained food production. Chapter 7 examines the role of conservation agriculture, and Chapter 8 presents the conclusions.
Annex 1 provides background information on the different soil organisms of importance in agriculture. Annex 2 provides details of the effects of organic matter on biological, chemical and physical soil properties.
Chapter 2. Organic matter decomposition and the soil food web
http://www.fao.org/docrep/009/a0100e/a0100e05.htm#TopOfPage
Soil organic matter
When plant residues are returned to the soil, various organic compounds undergo decomposition. Decomposition is a biological process that includes the physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules (Juma, 1998).
The continual addition of decaying plant residues to the soil surface contributes to the biological activity and the carbon cycling process in the soil. Breakdown of soil organic matter and root growth and decay also contribute to these processes. Carbon cycling is the continuous transformation of organic and inorganic carbon compounds by plants and micro- and macro-organisms between the soil, plants and the atmosphere (Figure 2)
![a0100e00[1]](http://www.econatics.co.za/wp-content/uploads/2015/01/a0100e001.gif)
| FIGURE 2 Carbon cycle |
Decomposition of organic matter is largely a biological process that occurs naturally. Its speed is determined by three major factors: soil organisms, the physical environment and the quality of the organic matter (Brussaard, 1994). In the decomposition process, different products are released: carbon dioxide (CO2), energy, water, plant nutrients and resynthesized organic carbon compounds. Successive decomposition of dead material and modified organic matter results in the formation of a more complex organic matter called humus (Juma, 1998). This process is called humification. Humus affects soil properties. As it slowly decomposes, it colours the soil darker; increases soil aggregation and aggregate stability; increases the CEC (the ability to attract and retain nutrients); and contributes N, P and other nutrients.
Soil organisms, including micro-organisms, use soil organic matter as food. As they break down the organic matter, any excess nutrients (N, P and S) are released into the soil in forms that plants can use. This release process is called mineralization. The waste products produced by micro-organisms are also soil organic matter. This waste material is less decomposable than the original plant and animal material, but it can be used by a large number of organisms. By breaking down carbon structures and rebuilding new ones or storing the C into their own biomass, soil biota plays the most important role in nutrient cycling processes and, thus, in the ability of a soil to provide the crop with sufficient nutrients to harvest a healthy product. The organic matter content, especially the more stable humus, increases the capacity to store water and store (sequester) C from the atmosphere.
The soil food web
The soil ecosystem (Box 1) can be defined as an interdependent life-support system composed of air, water, minerals, organic matter, and macro- and micro-organisms, all of which function together and interact closely.
The organisms and their interactions enhance many soil ecosystem functions and make up the soil food web. The energy needed for all food webs is generated by primary producers: the plants, lichens, moss, photosynthetic bacteria and algae that use sunlight to transform CO2 from the atmosphere into carbohydrates. Most other organisms depend on the primary producers for their energy and nutrients; they are called consumers.
BOX 1 Some functions of a healthy soil ecosystem
|
Soil life plays a major role in many natural processes that determine nutrient and water availability for agricultural productivity. The primary activities of all living organisms are growing and reproducing. By-products from growing roots and plant residues feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure and control the populations of soil organisms, both beneficial and harmful (pests and pathogens) in terms of crop productivity.
The living part of soil organic matter includes a wide variety of micro-organisms such as bacteria, viruses, fungi, protozoa and algae. It also includes plant roots, insects, earthworms, and larger animals such as moles, mice and rabbits that spend part of their life in the soil. The living portion represents about 5 percent of the total soil organic matter. Micro-organisms, earthworms and insects help break down crop residues and manures by ingesting them and mixing them with the minerals in the soil, and in the process recycling energy and plant nutrients. Sticky substances on the skin of earthworms and those produced by fungi and bacteria help bind particles together. Earthworm casts are also more strongly aggregated (bound together) than the surrounding soil as a result of the mixing of organic matter and soil mineral material, as well as the intestinal mucus of the worm. Thus, the living part of the soil is responsible for keeping air and water available, providing plant nutrients, breaking down pollutants and maintaining the soil structure.
The composition of soil organisms depends on the food source (which in turn is season dependent). Therefore, the organisms are neither uniformly distributed through the soil nor uniformly present all year. However, in some cases their biogenic structures remain. Each species and group exists where it can find appropriate food supply, space, nutrients and moisture (Plate 2). Organisms occur wherever organic matter occurs (Ingham, 2000). Therefore, soil organisms are concentrated: around roots, in litter, on humus, on the surface of soil aggregates and in spaces between aggregates. For this reason, they are most prevalent in forested areas and cropping systems that leave a lot of biomass on the surface.
Plate 2 Termites create their own living conditions near their preferred food sources. Inside the colony life is highly organized.
- MILLER
The activity of soil organisms follows seasonal as well as daily patterns. Not all organisms are active at the same time. Most are barely active or even dormant. Availability of food is an important factor that influences the level of activity of soil organisms and thus is related to land use and management (Figure 3). Practices that increase numbers and activity of soil organisms include: no tillage or minimal tillage; and the maintenance of plant and annual residues that reduce disturbance of soil organisms and their habitat and provide a food supply.
Different groups of organisms can be distinguished in the soil (Brussaard and Juma, 1995). Table 1 classifies them by size. Table 2 classifies them by function.
Decomposition process
Fresh residues consist of recently deceased micro-organisms, insects and earthworms, old plant roots, crop residues, and recently added manures.
Crop residues contain mainly complex carbon compounds originating from cell walls (cellulose, hemicellulose, etc.). Chains of carbon, with each carbon atom linked to other carbons, form the “backbone” of organic molecules. These carbon chains, with varying amounts of attached oxygen, H, N, P and S, are the basis for both simple sugars and amino acids and more complicated molecules of long carbon chains or rings. Depending on their chemical structure, decomposition is rapid (sugars, starches and proteins), slow (cellulose, fats, waxes and resins) or very slow (lignin).
| Figure 3 Fluctuations in microbial biomass at different stages of crop development in conventional agriculture compared with systems with residue retention and high organic matter input |
Source: Balota, 1996
TABLE 1 Classification of soil organisms
| Micro-organisms | Microflora | <5 µm | Bacteria Fungi |
| Microfauna | <100 µm | Protozoa Nematodes | |
| Macro-organisms | Meso-organisms | 100 µm – 2 mm | Springtails Mites |
| Macro-organisms | 2 – 20 mm | Earthworms Millipedes Woodlice Snails and slugs | |
| Plants | Algae | 10 µm | |
| Roots | > 10 µm |
Note: Clay particles are smaller than 2 µm. Source: adapted from Swift, Heal and Anderson, 1979.
TABLE 2 Essential functions performed by different members of soil organisms (biota)
| Functions | Organisms involved |
| Maintenance of soil structure | Bioturbating invertebrates and plant roots, mycorrhizae and some other micro-organisms |
| Regulation of soil hydrological processes | Most bioturbating invertebrates and plant roots |
| Gas exchange and carbon sequestration (accumulation in soil) | Mostly micro-organisms and plant roots, some C protected in large compact biogenic invertebrate aggregates |
| Soil detoxification | Mostly micro-organisms |
| Nutrient cycling | Mostly micro-organisms and plant roots, some soil- and litter-feeding invertebrates |
| Decomposition of organic matter | Various saprophytic and litter-feeding invertebrates (detritivores), fungi, bacteria, actinomycetes and other micro-organisms |
| Suppression of pests, parasites and diseases | Plants, mycorrhizae and other fungi, nematodes, bacteria and various other micro-organisms, collembola, earthworms, various predators |
| Sources of food and medicines | Plant roots, various insects (crickets, beetle larvae, ants, termites), earthworms, vertebrates, micro-organisms and their by-products |
| Symbiotic and asymbiotic relationships with plants and their roots | Rhizobia, mycorrhizae, actinomycetes, diazotrophic bacteria and various other rhizosphere micro-organisms, ants |
| Plant growth control (positive and negative) | Direct effects: plant roots, rhizobia, mycorrhizae, actinomycetes, pathogens, phytoparasitic nematodes, rhizophagous insects, plant-growth promoting rhizosphere micro-organisms, biocontrol agents Indirect effects: most soil biota |
During the decomposition process, microorganisms convert the carbon structures of fresh residues into transformed carbon products in the soil. There are many different types of organic molecules in soil. Some are simple molecules that have been synthesized directly from plants or other living organisms. These relatively simple chemicals, such as sugars, amino acids, and cellulose are readily consumed by many organisms. For this reason, they do not remain in the soil for a long time. Other chemicals such as resins and waxes also come directly from plants, but are more difficult for soil organisms to break down.
Humus is the result of successive steps in the decomposition of organic matter. Because of the complex structure of humic substances, humus cannot be used by many micro-organisms as an energy source and remains in the soil for a relatively long time.
Non-humic substances: significance and function
Non-humic organic molecules are released directly from cells of fresh residues, such as proteins, amino acids, sugars, and starches. This part of soil organic matter is the active, or easily decomposed, fraction. This active fraction is influenced strongly by weather conditions, moisture status of the soil, growth stage of the vegetation, addition of organic residues, and cultural practices, such as tillage. It is the main food supply for various organisms in the soil.
Carbohydrates occur in the soil in three main forms: free sugars in the soil solution, cellulose and hemicellulose; complex polysaccharides; and polymeric molecules of various sizes and shapes that are attached strongly to clay colloids and humic substances (Stevenson, 1994). The simple sugars, cellulose and hemicellulose, may constitute 5-25 percent of the organic matter in most soils, but are easily broken down by micro-organisms.
Polysaccharides (repeating units of sugar-type molecules connected in longer chains) promote better soil structure through their ability to bind inorganic soil particles into stable aggregates. Research indicates that the heavier polysaccharide molecules may be more important in promoting aggregate stability and water infiltration than the lighter molecules (Elliot and Lynch, 1984). Some sugars may stimulate seed germination and root elongation. Other soil properties affected by polysaccharides include CEC, anion retention and biological activity.
The soil lipids form a very diverse group of materials, of which fats, waxes and resins make up 2-6 percent of soil organic matter. The significance of lipids arises from the ability of some compounds to act as growth hormones. Others may have a depressing effect on plant growth.
Soil N occurs mainly (> 90 percent) in organic forms as amino acids, nucleic acids and amino sugars. Small amounts exist in the form of amines, vitamins, pesticides and their degradation products, etc. The rest is present as ammonium (NH4–) and is held by the clay minerals.
Compounds and function of humus
Humus or humified organic matter is the remaining part of organic matter that has been used and transformed by many different soil organisms. It is a relatively stable component formed by humic substances, including humic acids, fulvic acids, hymatomelanic acids and humins (Tan, 1994). It is probably the most widely distributed organic carbon-containing material in terrestrial and aquatic environments. Humus cannot be decomposed readily because of its intimate interactions with soil mineral phases and is chemically too complex to be used by most organisms. It has many functions (Box 2).
One of the most striking characteristics of humic substances is their ability to interact with metal ions, oxides, hydroxides, mineral and organic compounds, including toxic pollutants, to form water-soluble and water-insoluble complexes. Through the formation of these complexes, humic substances can dissolve, mobilize and transport metals and organics in soils and waters, or accumulate in certain soil horizons. This influences nutrient availability, especially those nutrients present at microconcentrations only (Schnitzer, 1986). Accumulation of such complexes can contribute to a reduction of toxicity, e.g. of aluminium (Al) in acid soils (Tan and Binger, 1986), or the capture of pollutants – herbicides such as Atrazine or pesticides such as Tefluthrin – in the cavities of the humic substances (Vermeer, 1996).
Humic and fulvic substances enhance plant growth directly through physiological and nutritional effects. Some of these substances function as natural plant hormones (auxines and gibberillins) and are capable of improving seed germination, root initiation, uptake of plant nutrients and can serve as sources of N, P and S (Tan, 1994; Schnitzer, 1986). Indirectly, they may affect plant growth through modifications of physical, chemical and biological properties of the soil, for example, enhanced soil water holding capacity and CEC, and improved tilth and aeration through good soil structure (Stevenson, 1994).
About 35-55 percent of the non-living part of organic matter is humus. It is an important buffer, reducing fluctuations in soil acidity and nutrient availability. Compared with simple organic molecules, humic substances are very complex and large, with high molecular weights. The characteristics of the well-decomposed part of the organic matter, the humus, are very different from those of simple organic molecules. While much is known about their general chemical composition, the relative significance of the various types of humic materials to plant growth is yet to be established.
Humus consists of different humic substances:
- Fulvic acids: the fraction of humus that is soluble in water under all pH conditions. Their colour is commonly light yellow to yellow-brown.
- Humic acids: the fraction of humus that is soluble in water, except for conditions more acid than pH 2. Common colours are dark brown to black.
- Humin: the fraction of humus that is not soluble in water at any pH and that cannot be extracted with a strong base, such as sodium hydroxide (NaOH). Commonly black in colour.
The term acid is used to describe humic materials because humus behaves like weak acids.
Fulvic and humic acids are complex mixtures of large molecules. Humic acids are larger than fulvic acids. Research suggests that the different substances are differentiated from each other on the basis of their water solubility.
Fulvic acids are produced in the earlier stages of humus formation. The relative amounts of humic and fulvic acids in soils vary with soil type and management practices. The humus of forest soils is characterized by a high content of fulvic acids, while the humus of agricultural and grassland areas contains more humic acids.
BOX 2 Humic substances retain nutrients available on demand for plantsFunctions of humus:
|
Chapter 3. Natural factors influencing the amount of organic matter
http://www.fao.org/docrep/009/a0100e/a0100e06.htm#TopOfPage
The transformation and movement of materials within soil organic matter pools is a dynamic process influenced by climate, soil type, vegetation and soil organisms. All these factors operate within a hierarchical spatial scale. Soil organisms are responsible for the decay and cycling of both macronutrients and micronutrients, and their activity affects the structure, tilth and productivity of the soil.
In natural humid and subhumid forest ecosystems without human disturbance, the living and non-living components are in dynamic equilibrium with each other (Figure 4). The litter on the soil surface beneath different canopy layers and high biomass production generally result in high biological activity in the soil and on the soil surface. Mollison and Slay (1991) distinguished the following five mechanisms:
- a continuous soil cover of living plants, which together with the soil architecture facilitates the capture and infiltration of rainwater and protects the soil;
- a litter layer of decomposing leaves or residues providing a continuous energy source for macro- and micro-organisms;
- the roots of different plants distributed throughout the soil at different depths permit an effective uptake of nutrients and an active interaction with microorganisms;
- the major period of nutrient release by micro-organisms coincides with the major period of nutrient demand by plants;
- nutrients recycled by deep-rooting plants and soil macrofauna and microfauna.
This equilibrium creates almost closed-cycle transfers of nutrients between soil and the vegetation adapted to such site conditions, resulting in almost perfect physical and hydric conditions for plant growth, i.e. a cool microclimate, increased evapotranspiration, good rooting conditions with good porosity and sufficient soil moisture. This facilitates water infiltration and prevents erosion and runoff. Thus, it results in clean water in the streams emanating from the area, a relatively smooth variation in streamflow during the year, and recharge of groundwater.
| FIGURE 4 Closed cycle |
In human-managed systems, the soil biological activity is influenced by the land use system, plant types and the management practices. Chapter 4 outlines the influence of land management practices. The environmental and edaphic factors that control the activity of soil biota, and thus the balance between accumulation and decomposition of organic matter in the soil, are described below.
Temperature
Several field studies have shown that temperature is a key factor controlling the rate of decomposition of plant residues. Decomposition normally occurs more rapidly in the tropics than in temperate areas. Ladd and Amato (1985) reported that, despite differences in plant material and climate patterns, the decomposition of leguminous materials in southern Australian sites followed the same pattern as that of ryegrass for sites in Nigeria and the United Kingdom (Jenkinson and Ayanaba, 1977), although the time scales were different. Reaction rates doubled for each increase of 8-9 °C in the mean annual air temperature. The relatively faster rate of decomposition induced by the continuous warmth in the tropics implies that high equilibrium levels of organic matter are difficult to achieve in tropical agro-ecosystems. Hence, large annual rates of organic inputs are needed to maintain an adequate labile soil organic matter pool in cultivated soils. Soils in cooler climates commonly have more organic matter because of slower mineralization (decomposition) rates.
Soil moisture and water saturation
Soil organic matter levels commonly increase as mean annual precipitation increases. Conditions of elevated levels of soil moisture result in greater biomass production, which provides more residues, and thus more potential food for soil biota.
Soil biological activity requires air and moisture. Optimal microbial activity occurs at near “field capacity”, which is equivalent to 60-percent water-filled pore space (Linn and Doran, 1984).
On the other hand, periods of water saturation lead to poor aeration. Most soil organisms need oxygen, and thus a reduction of oxygen in the soil leads to a reduction of the mineralization rate as these organisms become inactive or even die. Some of the transformation processes become anaerobic, which can lead to damage to plant roots caused by waste products or favourable conditions for disease-causing organisms. Continued production and slow decomposition can lead to very large organic matter contents in soils with long periods of water saturation (e.g. peat soils, and tea crops in India).
With the exception of the hyperhumid regions, the climates of vast areas of the humid, subhumid and semi-arid tropics are characterized by distinct wet and dry seasons. In the wet-dry tropics, large amounts of nitrate often occur in the surface soil during the first part of the rainy season (Greenland, 1958; Mueller-Harvey, Juo and Wild,1989). This accelerated nitrogen mineralization caused by a large increase in microbial activity is the result of the first few rains activating the labile soil organic matter.
Farmers who practise “slash and burn” agriculture often choose early planting in order to take advantage of this flush of inorganic N before it is lost through leaching and runoff. In these low-input systems, the amount of nitrate present in the soil during the early part of the rainy season is related closely to the organic matter content of the soil. N availability diminishes during the later part of the rainy season.
Soil texture
Soil organic matter tends to increase as the clay content increases. This increase depends on two mechanisms. First, bonds between the surface of clay particles and organic matter retard the decomposition process. Second, soils with higher clay content increase the potential for aggregate formation. Macroaggregates physically protect organic matter molecules from further mineralization caused by microbial attack (Rice, 2002). For example, when earthworm casts and the large soil particles they contain are split by the joint action of several factors (climate, plant growth and other organisms), nutrients are released and made available to other components of soil micro-organisms.
Under similar climate conditions, the organic matter content in fine textured (clayey) soils is two to four times that of coarse textured (sandy) soils (Prasad and Power, 1997).
Kaolinite, the main clay mineral in many upland soils in the tropics, has a much smaller specific surface and nutrient exchange capacity than most other clay minerals. Therefore, kaolinitic soils contain considerably fewer clay-humus complexes. In addition, the unprotected labile humic substances are vulnerable to decomposition under appropriate soil moisture conditions. Thus, high levels of organic matter are difficult to maintain in cultivated kaolinitic soils in the wet-dry tropics, because climate and soil conditions favour rapid decomposition. In contrast, organic matter can persist as organo-oxide complexes in soils rich in iron and aluminium oxides. Such properties favour the formation of soil microaggregates, typical of many fine-textured, oxide-rich, high base-status soils in the tropics (Uehara and Gilman, 1981). These soils are known for their low bulk density, high microporosity, and high organic-matter retention under natural vegetation, but also for their high phosphate fixation capacity on the oxides when used for crop production. Current knowledge suggests that whereas organic matter contributes to the dark colour of Vertisols (Coulombe, Dixon and Wilding, 1996), it is not considered important in determining either the development, robustness or resilience of structure in these soils (McGarry, 1996). Organic matter levels tend to be low in Vertisols; even as low as 10 g/ kg (Coulombe, Dixon and Wilding, 1996).
Parent material influences organic matter accumulation not only through its effect on soil texture. Soils developed from inherently rich material, such as basalt, are more fertile than soils formed from granitic material, which contains less mineral nutrients. Moreover, the former experience more organic matter accumulation because of abundant vegetative growth.
Topography
Organic matter accumulation is often favoured at the bottom of hills. There are two reasons for this accumulation: conditions are wetter than at mid- or upper-slope positions, and organic matter is transported to the lowest point in the landscape through runoff and erosion. Similarly, soil organic matter levels are higher on northfacing slopes (in the Northern Hemisphere) compared with south-facing slopes (and the other way around in the Southern Hemisphere) because temperatures are lower (Quideau, 2002).
Salinity and acidity
Salinity, toxicity and extremes in soil pH (acid or alkaline) result in poor biomass production and, thus in reduced additions of organic matter to the soil. For example, pH affects humus formation in two ways: decomposition, and biomass production. In strongly acid or highly alkaline soils, the growing conditions for micro-organisms are poor, resulting in low levels of biological oxidation of organic matter (Primavesi, 1984). Soil acidity also influences the availability of plant nutrients and thus regulates indirectly biomass production and the available food for soil biota. Fungi are less sensitive than bacteria to acid soil conditions.
Vegetation and biomass production
The rate of soil organic matter accumulation depends largely on the quantity and quality of organic matter input. Under tropical conditions, applications of readily degradable materials with low C:N ratios, such as green manure and leguminous cover crops, favour decomposition and a short-term increase in the labile nitrogen pool during the growing season. On the other hand, applications of plant materials with both large C:N ratios and lignin contents such as cereal straw and grasses (Figure 5) generally favour nutrient immobilization, organic matter accumulation and humus formation, with increased potential for improved soil structure development.
Plant constituents such as lignin and other polyphenols retard decomposition. In an experiment in southern Nigeria to compare management effects on soil organic matter accumulation, a three-year fallow with Guinea grass (Panicum maximum), which has a high lignin content, maintained a carbon level comparable to that under forest fallow. However, fallowing with leguminous species such as pigeon pea (Cajanus cajan) caused a significant decline in soil total C (Juo and Lal, 1977).
Palm and Sanchez (1990) reported that both the decomposition rate and the N-release patterns of three tropical legumes (Inga edulis, Cajanus cajan, and Erythrina spp.) were related to the amount of polyphenol compounds such as lignin in the leaf. Erythrina leaves had the lowest concentrations of polyphenols and the fastest decomposition rate of the three species studied.
Root turnover also constitutes an important addition of humus into the soil, and consequently it is important for carbon sequestration. In forests, most organic matter is added as superficial litter. However, in grassland ecosystems, up to two-thirds of organic matter is added through the decay of roots (Quideau, 2002).
| FIGURE 5 Composition of leaves and roots of leguminous and grass species |
Source: Primavesi, 1984
Chapter 4. Practices that influence the amount of organic matter
http://www.fao.org/docrep/009/a0100e/a0100e07.htm#TopOfPage
Human interventions that influence soil organic matter
Various types of human activity decrease soil organic matter contents and biological activity. However, increasing the organic matter content of soils or even maintaining good levels requires a sustained effort that includes returning organic materials to soils and rotations with high-residue crops and deep- or dense-rooting crops. It is especially difficult to raise the organic matter content of soils that are well aerated, such as coarse sands, and soils in warm-hot and arid regions because the added materials decompose rapidly. Soil organic matter levels can be maintained with less organic residue in finetextured soils in cold temperate and moist-wet regions with restricted aeration.
Practices that decrease soil organic matter
Any form of human intervention influences the activity of soil organisms (Curry and Good, 1992) and thus the equilibrium of the system. Management practices that alter the living and nutrient conditions of soil organisms, such as repetitive tillage or burning of vegetation, result in a degradation of their microenvironments. In turn, this results in a reduction of soil biota, both in biomass and diversity. Where there are no longer organisms to decompose soil organic matter and bind soil particles, the soil structure is damaged easily by rain, wind and sun. This can lead to rainwater runoff and soil erosion (Plate 3), removing the potential food for organisms, i.e. the organic matter of the topsoil. Therefore, soil biota are the most important property of the soil, and “when devoid of its biota, the uppermost layer of earth ceases to be soil” (Lal, 1991).
Plate 3 Severe soil erosion removes the potential energy source for soil microbes, resulting in the death of the microbial population and thus of the soil itself.
- MUELLER
| FIGURE 6 Open cycle system |
The factors leading to reduction in soil organic matter in an open cycle system (Figure 6) can be grouped as factors that result in:
- a decrease in biomass production;
- a decrease in organic matter supply;
- increased decomposition rates.
Decrease in biomass production
Replacement of perennial vegetation
A consequence of clearing forest for agriculture is the disappearance of the litter layer, with a consequent reduction in the numbers and variety of soil organisms. While many temperate forest species appear to adapt well to grassland (Curry and Good, 1992), the effects of deforestation in the tropics appear to be more marked (Plate 4). Studies have shown that as soil biodiversity declines, adapted species may take over from the indigenous species and the composition may change drastically.
Soil macrofaunal biomass and population density fell to 6 and 17 percent, respectively, in cultivated plots, compared with primary forest in Peruvian Amazonia (Lavelle and Pashanasi, 1989). In Suriname, the number of animals per square metre has fallen to 36 percent and the diversity of species has fallen to 28 percent compared with primary forest (Van der Werff, 1990). The indigenous species have largely disappeared, but adapted species have been available for recolonization. The composition of the macrofaunal community has changed drastically (Figure 7).
| FIGURE 7 Composition of soil macrofauna under primary forest, fallow, kudzu and grass vegetation |
Source: Van der Werff, 1990
Plate 4 Clearance of primary forests often leads to rapid mineralization of organic matter. This sandy soil used to sustain a tropical forest.
A.G. BOT
Replacement of mixed vegetation with monoculture of crops and pastures
The simplification of vegetation and the disappearance of the litter layer under grassland and monocrop production systems lead to a decrease in faunal diversity. Although root systems (especially of grasses) can be extensive and explore vast areas of soil, the root exudates from one single crop will attract only a few different microbial species. This in turn will affect the predator diversity. The more opportunistic pathogen species will be able to acquire space near the crop and cause harm. Continuous cultivation and grazing also leads to compaction of soil layers, which in turn affects the circulation of air. Anaerobic conditions in the soil stimulate the growth of different micro-organisms, resulting in more pathogenic organisms.
High harvest index
One of the consequences of the green revolution was the replacement of indigenous varieties of species with high-yielding varieties (HYVs). These HYVs often produce more grain and less straw, compared with locally developed varieties; the harvest index of the crop (ratio of grain to total plant mass aboveground) is increased. From a production point of view, this is a logical approach. However, this is less desirable from a conservation point of view. Reduced amounts of crop residues remain after harvest for soil cover and organic matter, or for grazing of livestock (which results in manure). Moreover, where animals graze the residues, even less remains for conservation purposes.
Use of bare fallow
Traditionally, a fallow period is used after a period of crop production to give the land some “rest” and to regenerate its original state of productivity. Usually, this is necessary in production systems that have drawn down the nutrient supply and altered the soil biota significantly, such as in slash-and-burn systems or conventional tillage systems.
Some farmers use bare fallow to regenerate their lands. However, apart from spontaneous weed growth, this means there is no energy source for the soil biota present on the land. Instead of recovering the soil food web, the soil organic matter is degraded further and the lack of cover can result in severe erosion and runoff when the rains start after the dry season.
Plate 5 The burning of residues is a common practice for clearing land, both in slash-and-burn systems and in more intensive agricultural production systems.
A.J. BOT
Decrease in organic matter supply
Burning of natural vegetation and crop residues
The burning of maize, rice and other crop residues in the field is a common practice (Plate 5). Residues are usually burned to help control insects or diseases or to make fieldwork easier in the following season. Burning destroys the litter layer and so diminishes the amount of organic matter returned to the soil. The organisms that inhabit the surface soil and litter layer are also eliminated. For future decomposition to take place, energy has to be invested first in rebuilding the microbial community before plant nutrients can be released. Similarly, fallow lands and bush are burned before cultivation. This provides a rapid supply of P to stimulate seed germination. However, the associated loss of nutrients, organic matter and soil biological activity has severe long-term consequences.
Overgrazing
There is a tendency throughout the world to overstock grazing land above its carrying capacity. Cows, draught animals and small ruminants graze on communal grazing areas and on roadsides, stream banks and other public land (Plate 6). Overgrazing destroys the most palatable and useful species in the plant mixture and reduces the density of the plant cover, thereby increasing the erosion hazard and reducing the nutritive value and the carrying capacity of the land.
Removal of crop residues
Many farmers remove residues from the field for use as animal feed and bedding or to make compost (Plate 7). Later, these residues return to contribute to soil fertility as manures or composts. However, residues are sometimes removed from the field and not returned. This removal of plant material impoverishes the soil as it is no longer possible to recycle the plant nutrients present in the residues.
Plate 6 Animal at pasture in the area around Asmara.
- FAIDUTTI/FAO/19452
Increased decomposition rates
Tillage practices
Tillage is one of the major practices that reduces the organic matter level in the soil. Each time the soil is tilled, it is aerated. As the decomposition of organic matter and the liberation of C are aerobic processes, the oxygen stimulates or speeds up the action of soil microbes, which feed on organic matter.
This means that:
- When ploughed, the residues are incorporated in the soil together with air and come into contact with many micro-organisms, which accelerates the carbon cycle. The decomposition is faster, resulting in the formation of less stable humus and an increased liberation of CO2 to the atmosphere, and thus a reduction in organic matter.
- The residues on the soil surface slow the carbon cycle because they are exposed to fewer micro-organisms and thus wane more slowly, resulting in the production of humus (which is more stable), and liberating less CO2 to the atmosphere.
Plate 7 Removed crop residues cannot serve as food for soil organisms.
FAO
TABLE 3 Tillage induced flush of decomposition of organic matter
| Type of tillage | Organic matter lost in 19 days (kg/ha) |
| Mouldboard plough + disc harrow (2x) | 4 300 |
| Mouldboard plough | 2 230 |
| Disc harrow | 1 840 |
| Chisel plough | 1 720 |
| Direct seeding | 860 |
Source: Glanz, 1995
In terms of short-term organic matter loss, the more a soil is tilled, the more the organic matter is broken down (Table 3). There are also longer-term losses, attributed to repeated, annual cultivation. Cropping systems that return little residue to the soil accelerate this decline. Many modern cropping systems combine frequent tillage with small amounts of residue, with resultant reductions in the organic matter content of many soils. Historically, manure application (from farm livestock) was common, and it was a dynamic way of maintaining organic matter levels despite repeated cultivation and low residue returns to the soil. Increased on-farm mechanization has reduced livestock numbers, so this source of organic material has been reduced considerably.
Organic matter production and conservation is affected dramatically by conventional tillage, which not only decreases soil organic matter but also increases the potential for erosion by wind and water (Plate 8). The impact occurs in many ways:
- Ploughing leaves no residues on the soil surface to lessen the impact of rain.
- Ploughing reduces the quantity of food sources for earthworms and disturbs their burrows and living space, hence populations of certain species decrease drastically. Moreover, reduction of earthworm numbers reduces their impact, through burrowing, in increasing porosity and aeration (particularly continuous macropores) and lowers their ability to bury and incorporate plant residues, which facilitates rapid decomposition of organic matter.
- Tillage by repeated hoeing or discing smoothes the surface and destroys natural soil aggregates and channels that connect the surface with the subsoil, leaving the soil susceptible to erosion. Old root channels and earthworm holes are eliminated, as are the cracks between natural aggregates. The large pores, the ones destroyed by conventional tillage practices, are necessary to conduct water into the soil during rainfall.
- The development of a plough pan or hoe pan, a layer of compacted soil resulting from smearing action at the bottom of the plough or hoe, may retard both root penetration and water infiltration.
- Ploughing or discing under dry conditions exacerbates the pulverization of the soil, causing the soil surface to crust more easily, leading to greater water runoff and erosion. This is exacerbated by reduced soil surface roughness, which leaves few depressions for temporary storage of water during intense storms.
- Increased runoff during rainstorms may also increase the possibility of drought stress later in the season, because water that runs off the field does not infiltrate into the soil to remain available to plants.
In some circumstances, imbalances of certain soil organisms can disrupt soil structure and processes, e.g. certain earthworm species in rice fields or pastures.
Plate 8 Intensive soil tillage makes the land vulnerable to erosive processes, as the organic matter is lost through increased oxidation in the soil, the upper subsoil is compact, and the loosened topsoil can more readily wash away.
- JONES/FAO/19862
Drainage
Decomposition of organic matter occurs more slowly in poorly aerated soils, where oxygen is limiting or absent, compared with well-aerated soils. For this reason, organic matter accumulates in wet soil environments. Soil drainage is determined strongly by topography – soils in depressions at the bottom of hills tend to remain wet for extended periods of time because they receive water (and sediments) from upslope. Soils may also have a layer in the subsoil that inhibits drainage, again exacerbating waterlogging and reduction in organic matter decomposition. In a permanently waterlogged soil, one of the major structural parts of plants, lignin, does not decompose at all. The ultimate consequence of extremely wet or swampy conditions is the development of organic (peat or muck) soils, with organic matter contents of more than 30 percent. Where soils are drained artificially for agricultural or other uses, the soil organic matter decomposes rapidly.
Fertilizer and pesticide use
Initially, the use of fertilizer and pesticides enhances crop development and thus production of biomass (especially important on depleted soils). However, the use of some fertilizers, especially N fertilizers, and pesticides can boost micro-organism activity and thus decomposition of organic matter. The chemicals provide the microorganisms with easy-to-use N components. This is especially important where the C: N ratio of the soil organic matter is high and thus decomposition is slowed by a lack of N.
Practices that increase soil organic matter
Increased concern about the environmental and economic impacts of conventional crop production has stimulated interest in alternative systems. Central to such systems is the need to promote and maintain soil biological processes and minimize fossil fuel inputs in the form of fertilizers, pesticides and mechanical cultivation. All activities aimed at the increase of organic matter in the soil (Box 3) help in creating a new equilibrium in the agro-ecosystem.
For a system of natural resource management to be balanced, and thus sustainable, it must be able to withstand sharp climatic fluctuations, and to evolve steadily in response to social changes and changes in the costs and availability of inputs of land, labour and knowledge. The more diverse and complex an agricultural system is, the more stable and sustainable it will be in the face of unpredictable vagaries of climate and market. Thus, annual crops, woody perennials and nonwoody perennials may be combined in various ways with livestock or trees, or both, in what are now commonly called agrosilvipastoral systems.
Different approaches are required for different soil and climate conditions. However, the activities will be based on the same principle: increasing biomass production in order to build active organic matter. Active organic matter provides habitat and food for beneficial soil organisms that help build soil structure and porosity, provide nutrients to plants, and improve the water holding capacity of the soil.
Several cases have demonstrated that it is possible to restore organic matter levels in the soil (Figure 8). Activities that promote the accumulation and supply of organic matter, such as the use of cover crops and refraining from burning, and those that reduce decomposition rates, such as reduced and zero tillage, lead to an increase in the organic matter content in the soil (Sampson and Scholes, 2000).
BOX 3 Ways to increase organic matter contents of soils
|
| FIGURE 8 Evaluation of the organic matter content of a soil in Paraná |
Source: adapted from Derpsch, 1997.
Increased biomass production
Increased water availability for plants: water harvesting and irrigation
In dry conditions, water may be provided through irrigation or water harvesting. The increased water availability enhances biomass production, soil biological activity and plant residues and roots that provide organic matter.
The concept of water harvesting includes various technologies for runoff management and utilization. It involves capture of runoff (in some cases through treating the upstream capture area), and its concentration on a runon area for use by a specific crop (annual or perennial) in order to enhance crop growth and yields, or its collection and storage for supplementary irrigation or domestic or livestock purposes. The objective of designing a water harvesting system is to obtain the best ratio of the area yielding runoff to either the area where runoff is being directed or the capacity of the storage structure (volume of water collected). In this way, the water captured for crop production during runoff periods can be stored either directly in the soil for subsequent use by plants or in small farm reservoirs or collection tanks (Plate 9). This aids stabilization of crop production by enhancing soil moisture availability or allowing irrigation during a dry period within the rainy season or by extending crop production into the dry season. Some factors to be considered regarding these runoff farming systems and reservoirs include: site selection, watershed size and condition, rainfall distribution and runoff, and water requirements of crops. Where a minimum water depth of about 1 m can be maintained in a reservoir, fish can be raised to provide additional food (FAO, 1984).
Numerous water harvesting systems have been developed over the centuries, especially in arid areas. The principle of collecting runoff for crop production is also inherent to many other soil and water conservation technologies that apply the concept of runoff and runon areas at a microwatershed level, such as negarims, trapezoidal or “eyebrow” bunds and tied ridges.
Plate 9 Hollows are dug in the ground to gather water during the rainy season.
- BALDERI/FAO/18852
Balanced fertilization
Where the supply of nutrients in the soil is ample, crops are more likely to grow well and produce large amounts of biomass. Fertilizers are needed in those cases where nutrients in the soil are lacking and cannot produce healthy crops (FAO, 2000) and sufficient biomass. Most soils in sub-Saharan Africa (SSA) are deficient in P. P is required not only for plant growth but also for N fixation. Unbalanced fertilization, for example mainly with N, may result in more weed competition, higher pest incidence and loss of quality of the product. Unbalanced fertilization eventually leads to unhealthy plants. Therefore, fertilizers should be applied in sufficient quantities and in balanced proportions. The efficiency of fertilizer use will be high where the organic matter content of the soil is also high. In very poor or depleted soils, crops use fertilizer applications inefficiently. When soil organic matter levels are restored, fertilizer can help maintain the revolving fund of nutrients in the soil by increasing crop yields and, consequently, the amount of residues returned to the soil.
Cover crops
Growing cover crops is one of the best practices for improving organic matter levels and, hence, soil quality. The benefits of growing cover crops include:
- They prevent erosion by anchoring soil and lessening the impact of raindrops.
- They add plant material to the soil for organic matter replenishment.
- Some, e.g. rye, bind excess nutrients in the soil and prevent leaching.
- Some, especially leguminous species, e.g. hairy vetch, fix N in the soil for future use.
- Most provide habitat for beneficial insects and other organisms.
- They moderate soil temperatures and, hence, protect soil organisms.
A range of crops can be used as vegetative cover, e.g. grains, legumes and oil crops. All have the potential to provide great benefit to the soil. However, some crops emphasize certain benefits; a useful consideration when planning a rotation scheme. It is important to start the first years with (cover) crops that cover the surface with a large amount of residues that decompose slowly (because of the high C:N ratio). Grasses and cereals are most appropriate for this stage, also because of their intensive rooting system, which improves the soil structure rapidly.
In the following years, when soil health has begun to improve, legumes can be incorporated in the rotation. Leguminous crops enrich the soil with N and their residues decompose rapidly because of their low C:N ratio. Later, when the system is stabilized, it is possible to include cover crops with an economic function, e.g. livestock fodder.
The selection of cover crops should depend on the presence of high levels of lignin and phenolic acids. These give the residues a higher resistance to decomposition and thus result in soil protection for a longer period and the production of more stable
| FIGURE 9 Reduction of dry matter of different cover crops |
Source: Ruedell, 1995 humus.
Another determining factor in the dynamics of residue composition is the biochemical composition of the residues. Depending on species, their chemical components and the time and way of managing them, there will be differences in decomposition rates (Figure 9). The grain species (oats and wheat) show more resistance than common vetch (legume) to decomposition. The latter has a lower C:N ratio and a lower lignin content and is thus subject to a rapid decomposition.
Agricultural production systems in which residues are left on the soil surface, such as direct seeding and the use of cover crops, stimulate the development and activity of soil fauna at many levels.
The term green manure is often used to indicate the same plant species that are used as cover crops. However, green manure refers specifically to a crop in the rotation grown for incorporation of the non-decomposed vegetative matter in the soil. While this practice is used specifically to add organic matter, this is not the most effective use of organic matter (especially in hot climates) for two reasons:
- Mechanical disturbance of the soil should be avoided as much as possible.
- When biomass is incorporated in the soil all at one time, there is a short period of high microbial activity in decomposing the material. This results in the sudden release of a large quantity of nutrients that cannot be captured by the seedlings of the following crop and is thus lost from the system.
In general, the greater the production of green manure or crop biomass, the greater is the microbial, mesofauna and macrofauna population of the soil – from fungi and micro-organisms to earthworms and termites. The dynamics of surface residue decomposition depend inter alia on the activity of micro-organisms and also on soil mesofauna and macrofauna. The macrofauna consists mainly of earthworms, beetles, termites, ants, millipedes, spiders, snails and slugs. These organisms help integrate the residues into the soil and improve soil structure, porosity, water infiltration, and through-flow through the creation of burrows, ingestion and secretions.
The natural incorporation of cover-crop and weed residues from the soil surface to deeper layers in the soil by soil macrofauna is a slow process. The activity of microorganisms is regulated by the activity of the macrofauna, because the latter provide them with food and air through their bioturbation activities. In this way, nutrients are released slowly and can provide the crop with nutrients over a longer period. At the same time, the soil is covered for a long time by the residues and is protected against the impact of rain and sun.
Improved vegetative stands
In many places, low plant densities limit crop yields. Wide plant spacing is often practised as “a way to return power to the soil” or “to give the soil some rest”, but in reality it is an indicator that the soil is impoverished. Plant spacing is usually determined by farmers in relation to soil fertility and available water or expected rainfall (unless standard recommendations are enforced by extension). This means that plants are often spaced widely on depleted soils in arid and semi-arid regions with a view to ensuring an adequate provision of plant nutrients and water for all plants.
However, it is important to maintain the recommended plant spacing in order to optimize biomass production and rooting density and, hence, organic matter for food, moisture retention and habitat for soil organisms. Once the crop is established, reduced sunlight between closer crop rows may also reduce regrowth of weeds.
| BOX 4 Planting pitsPlanting pits achieve fast rehabilitation of severely degraded land, especially in a semi-arid climate where a short fallow period of natural grass growth (2-6 years after 2-3 years under crops) cannot be expected to maintain or restore the land’s agricultural productivity (FAO, 1994).An example of the rapid restoration of productivity of degraded land is an indigenous method in the Sahel region called “zaï” (FAO, 1994). During the dry season, farmers dig out pits 15 cm deep and 40 cm in diameter every 80 cm, tossing the earth downhill. The dry desert Harmattan wind blows various organic residues into the excavated pits. The organic materials are consumed quickly by termites, which excavate tunnels through the crusted surface, allowing the first rains to soak down deep, out of danger of direct evaporation.Two weeks before the onset of the rains, farmers spread one or two handfuls of dry dung (1- 2.5 tonnes/ha) in the bottom of the pits and cover it with earth to prevent the rains from eroding away the organic matter.Millet is sown into the pits at the onset of the rainy season. As the first rains wash over the surface crust (of the degraded land), the basins capture this runoff (enough to soak a pocket of soil up to 1 m in depth). The sown seeds germinate, break up the slaked surface crust and send roots down to the deeper stores of both water and nutrients (recycled by the termites).At harvest time, stalks are cut at a height of 1 m and left in situ to reduce wind-speed and trap windborne organic matter. In the second year, the farmer either digs new basins between the first ones and dresses them with manure, or pulls up the stubble and sows again in the old basins. Stubble clumps laid between basins are in turn used as a food source by termites. |
Planting pits are a way of increasing biomass production and crop yields on severely degraded land in semi-arid conditions. Rainfall is concentrated near the plants, and soil faunal activity and organic matter accumulation are concentrated in the planting pits (Box 4 and Plate 10). Planting pits have been introduced successfully in Zambia as a conservation practice for smallholder farmers, who do not have fertilizers or tractor services available to them.
Plate 10 Half-moons around newly planted Acacia seedlings catch and retain rainwater.
T.F. SHAXSON
Agroforestry and alley cropping
Agroforestry is a collective name for land-use systems where woody perennials (trees, shrubs, palms, etc.) are integrated in the farming system (FAO, 1989). Alley cropping is an agroforestry system in which crops are grown between rows of planted woody shrubs or trees. These are pruned during the cropping season to provide green manure and to minimize shading of crops (FAO, 1993).
Agroforestry covers a wide range of systems (Box 5) combining food crops, forestry and pasture species in different ways (agrosilviculture, silvipasture, agrosilvipasture and multipurpose forest production). There are two different approaches to agroforestry. One uses agricultural crops or pasture as a transitional means of utilizing the land until forest plantations are fully established. The other is to integrate trees and shrubs permanently into the crop or animal production system, to the benefit of both crop production and land resource protection. Thus, agroforestry encompasses many traditional land-use systems such as home gardens, shifting cultivation and bush fallow systems (FAO, 1989).
| BOX 5 Examples of agroforestry systems worldwidePoro (Erythrina poeppigiana) has been grown extensively in coffee plantations in Costa Rica for shade, soil enrichment, live mulching and live fences.Albizzia spp. have been used in tea plantations in many Asian countries.In Indonesia, leucaena (Leucaena leucocephala) has been planted as contour hedges on hillsides for erosion control, soil improvement and green mulch. It is estimated that some 20 000 ha of undulating land have been converted to these systems.In West Africa and Rwanda, many farmers use trees, fruit trees, bushes and grasses planted with agricultural crops on their farms.Many coconut plantations in the Caribbean are partly planted with bananas or used as pastures. Some small farms in Jamaica plant coconuts, banana and citrus together.(FAO, 1989) |
Alley cropping can be considered an improved bush fallow system. Small trees or shrubs are planted in cropland in rows, preferably along the contour (even where east-west orientation of the rows may minimize shading of crops). The optimal spacing between rows depends on: slope; soil type and its susceptibility to erosion; rainfall; crop species; and the soil and crop management system (FAO, 1995).
Besides adding organic matter to the system, perennial trees and shrubs recycle plant nutrients from deeper soil layers through their rooting system. Through litter and pruning, these can be used again by annual crops. Probably the most important contribution of perennials in a production system lies in the fact that throughout the whole year their roots excrete root exudates and decaying root cells, which in turn are used as an energy source by soil microorganisms. The food web in the soil is maintained, even during dry seasons when no annual crops are grown. The result is that soil biota are in place to provide the crop with nutrients at the beginning of the next cropping season.
Direct seeding is the easiest and cheapest way of establishing hedgerows around fields or in the fields (alleys). However, emerging seedlings may not be able to compete with weeds without additional care. Therefore, starting plant growth in a nursery and transplanting may be necessary for some species. Other species may be established by cuttings. With good establishment, the plants will be better able to withstand both dry spells and browsing by livestock. Crucial to a successful establishment of the hedgerow is that the selected plants should be tall enough to outgrow the weeds at the time of the first crop harvest.
Plate 11 Agroforestry plot with suboptimal stand of maize because of shading by trees.
- MUELLER
During the cropping season, hedgerow pruning is needed in order to avoid shading of the crop (Plate 11). The timing, frequency and extent of pruning depend on the species used and the season. As a general rule, the lower the hedgerows and the taller the crop, the less frequently is pruning required. Fastgrowing plants such as Leucaena leucocephala and Gliricidia sepium may require pruning every six weeks during the cropping season. They are often pruned to a height of about 50 cm. Care must be exercised as too frequent pruning can result in tree dieback.
The integration of trees and woody shrubs into the cropping system offers additional uses and many benefits, as mentioned by farmers using the Quezungual system in Honduras (Plate 12 and Box 6). However, farmers with short-term land tenure may not be interested in these benefits. Furthermore, the plantation of trees sometimes has an effect on the land tenure status; therefore tenants may not be allowed to establish trees on agricultural land. Agroforestry systems can also inhibit mechanization and may need increased labour inputs, especially for hedgerow pruning.
BOX 6 Farmers’ perceptions of the Quezungual system: benefits and disadvantagesThe Quezungual system has many benefits according to local farmers:
Disadvantages mentioned by the farmers are:
(FAO, 2001) |
Plate 12 Example of the “Quezungual” system, an indigenous agroforestry system.
NELSON GONZALEZ
The hedgerow species have to be selected carefully in order to avoid negative impacts on crop production because of the complex relationships (competition for light, water and nutrients, allelopathy, occurrence of pest and diseases, etc.) that are inherent to agroforestry systems. Many farmers may consider the hedgerows as not useful, especially where their positive effects are not secure or visible. Where livestock are allowed to graze freely, it can be difficult to establish hedgerows without taking special measures to protect the young plants. Fencing or control of grazing animals may require collective efforts and agreement by the local community.
The increased labour requirement, the reduced cropped area and the difficulty of mechanization may make alley cropping uneconomic unless the hedgerow species produce direct benefits such as fruits, fuelwood or poles/timber for construction purposes (in addition to the nutrient recycling and erosion control effects).
Plate 13 Afforestation of “badlands” in the Niger.
FAO
Reforestation and afforestation
Afforestation means the establishment of a forest on land that has not grown trees recently. It can serve two principal soil and water conservation purposes: protection of erosion-prone areas, and revegetation and rehabilitation of degraded land (Plate 13). Afforestation is specifically used to provide protective cover in vulnerable, steep and mountainous areas. Afforestation helps to replenish timber resources and provide fuelwood and fodder (FAO, 1979).
The establishment of a forest cover under good management is an effective means of increasing organic matter production. However, the land must have the productive capacity to support an appropriate forest type, which differs according to climate, soil, slope and the specific purpose of the forest (timber production, livestock grazing, etc.). Therefore, the choice of species and the selection of an appropriate site are of particular importance for successful afforestation.
The procurement of adequate quantities of good quality seed of the species and provenances (adapted varieties) required is a prerequisite for any afforestation effort. However, it is often difficult to find suitable and reliable sources of such seeds.
A number of species require special pre-treatment of the seed or seedling in order to achieve satisfactory germination and uniform stand. Such treatment may consist of soaking the seed in water for varying lengths of time, alternate soaking and drying, scarifying or chipping the seed coat to render it permeable to water, plunging the seed into boiling water or even boiling it for a short time. Some tree seedlings may need a mycorrhizal treatment when planted in soils that are deprived of associated mycorrhizae species as well as rhizobium species (e.g. Casuarina in Senegalese sandy soils). The aim is to ensure that good numbers of plants germinate and that germination after sowing is both rapid and uniform (FAO, 1974).
Afforestation can be achieved by direct sowing or replanting young plants from a nursery. The main advantage of direct sowing is the reduced cost. However, this is usually much less reliable and is only justified where:
- seed is plentiful and cheap;
- adequate germination under field conditions can be relied on;
- the seedlings send down a deep tap root rapidly and are able to withstand adverse climatic conditions in the time after germination;
- the rate of growth is sufficiently fast to make a prolonged period of tending and weeding unnecessary.
Regeneration of natural vegetation
Regeneration of natural grasslands and forest areas increases biomass production and improves the plant species diversity, resulting in more diverse soil biota and other associated beneficial organisms. Natural regeneration may be more reliable where land is not very productive. In some cases, natural regeneration of a given area may lead to the infestation of plots by weeds. Increasingly, natural vegetation is being recognized for its multipurpose benefits, for example, fuelwood, fibre, biocontrol (e.g. neem) and medicinal species, as well as restoration of soil fertility (Acacia albida and other leguminous species) and habitats for various beneficial species (pollinators and natural enemies) as well as wildlife.
Increased organic matter supply
Protection from fire
Burning affects organic matter recycling significantly. Fire destroys almost all organic materials on the land surface except for tree trunks and large branches. In addition, the surface soil is sterilized, loses part of its organic matter, the population of soil microfauna and macrofauna is reduced, and no ready-to-use organic matter is available for rapid restoration of the populations. However, this practice is widely used (e.g. in Africa) in order to enhance pasture regrowth for livestock (using residual P), to control pests and diseases, and even to catch small animals for food.
A specific and difficult case is the burning of sugar cane before harvesting. It has both a technical dimension (CO2 and greenhouse gas emissions, mechanization of harvest, sugar content, etc.) and a social dimension (manual cutting, source of survival resources for poor/landless workers). The damage depends on fire intensity, which is a function of vegetation type and climate conditions and frequency. The costs and benefits of burning and the methods to minimize harmful effects need to be identified with local populations.
Crop residue management
In systems where crop residues are managed well, they:
- add soil organic matter, which improves the quality of the seedbed and increases the water infiltration and retention capacity of the soil, buffers the pH and facilitates the availability of nutrients;
- sequester (store) C in the soil;
- provide nutrients for soil biological activity and plant uptake;
- capture the rainfall on the surface and thus increase infiltration and the soil moisture content;
- provide a cover to protect the soil from being eroded;
- reduce evaporation and avoid desiccation from the soil surface.
Depending on the nature of the following crop, decisions are made as to whether the residues should be distributed evenly over the field or left intact, e.g. where climbing cover crops (e.g. mucuna) use the maize stalks as a trellis.
An even distribution of residues: (i) provides homogenous temperature and humidity conditions at sowing time; (ii) facilitates even sowing, germination and emergence; (iii) minimizes the development of pests and diseases; and (iv) reduces the emergence of weeds through allelopathic effects.
The most appropriate method for managing crop residues depends on the purpose of the crop residues and the experience and equipment available to the farmer. Where the aim is to maintain a mulch over the soil for as long as possible, the biomass is best managed using a knife roller, chain or sledge in order to break it down but not kill it (Plate 14). Where the decomposition process should commence immediately in order to release nutrients, the residues should be slashed or mown and some N applied because dry residues have a high C:N ratio. However, in order to avoid nitrate emission, urea should not be broadcast on the surface but injected where possible.
Plate 14 Crop residue management by using a knife roller.
A.J. BOT
Utilizing forage by grazing rather than by harvesting
In many places, there is competition for the use of crop residues that can be used as fodder, for roofing, artisan handicrafts, etc. Where residues are to be used for animal feed, either the animals graze the residues directly, or they are stall- or kraal-fed.
Removal of the residues from the field can lead to a considerable loss of organic matter where animal manure is not returned to the field. By controlled grazing, the animal manure is returned in the field without a high labour input.
The experience of Guaymango, El Salvador, demonstrates that it is possible to achieve successful integration of crop and livestock components without creating competition in the allocation of crop residues (Vieira and Van Wambeke, 2002). The amount of residues produced by the system is enough to serve both as soil cover and as fodder for livestock (Choto and Saín, 1993), mainly because of the use of local sorghum varieties (instead of HYVs) that have a high straw/grain ratio (Choto, Saín and Montenegro, 1995). As farmers value crop residues as soil cover, a fodder market has developed where grazing rights, number of cattle and duration of grazing are traded.
In the northern zone of the United Republic of Tanzania, farmers have found a compromise between using the residues for grazing or soil cover, albeit one that is rather labour intensive. They separate the palatable and non-palatable parts of the crop residues. They use the non-palatable parts to cover the soil and act as food for soil organisms, while they feed the palatable parts to cattle and goats that are kept close to the homestead (Plate 15).
Integrated pest management
As with balanced fertilization, proper pest and disease management results in healthy crops. Healthy crops produce optimal biomass, which is necessary for organic matter production in the soil. Diversified cropping and mixed crop-livestock systems enhance biological control of pests and diseases through species interactions. Through integrated production and pest management farmers learn how to maintain a healthy environment for their crops. They learn to examine their crops regularly in order to observe ratios of pests to natural enemies (beneficial predators) and cases of damage, and on that basis to make decisions as to whether it is necessary to use natural treatments (using local products such as neem or tobacco) or chemical treatments and the required applications.
Plate 15 Farmer giving fodder to cattle
- JONES/FAO/1937
Applying animal manure or other carbon-rich wastes
Any application of animal manure, slurry or other carbon-rich wastes, such as coffee-berry pulp, improves the organic matter content of the soil. In some cases, it is better to allow a period of decomposition before application to the field. Any addition of carbon-rich compounds immobilizes available N in the soil temporarily, as micro-organisms need both C and N for their growth and development. Animal manure is usually rich in N, so N immobilization is minimal. Where straw makes up part of the manure, a decomposition period avoids N immobilization in the field.
Plate 16 Preparation of compost made from discarded bits of fish and village waste. The compost plant is run by a cooperative of young people.
- BALDERI/FAO/18775
Compost
Composting is a technology for recycling organic materials in order to achieve enhanced agricultural production. Biological and chemical processes accelerate the rate of decomposition and transform organic materials into a more stable humus form for application to the soil. Composting proceeds under controlled conditions in compost heaps and pits (Müller-Sämann, 1986).
Compost heaps should have a minimum size of 1 m3 and are suitable for more humid environments where there is potential for watering the compost. Compost pits (Plate 16) should be no deeper than 70 cm and should be underlain with rough material for good aeration of the compost. Pits are suitable for drier environments where the compost may desiccate (Müller-Sämann, 1986). Dry composting relies on covering the compost with soil and creating an anaerobic environment. However, this is a slower process than the more usual moist aerobic process. The ratio of C to N in the compost pile is important for optimizing microbial activity. Thus, a mixture of soft, green and brown, tougher material is used. Ash and phosphate rock are often added to accelerate the process.
Composting can complement certain crop rotations and agroforestry systems. It can be used efficiently in planting pits and nurseries. It is very similar in composition to soil organic matter. It breaks down slowly in the soil and is very good at improving the physical condition of the soil (whereas manure and sludge may break down fairly quickly, releasing a flush of nutrients for plant growth). In many circumstances, it takes time to rejuvenate a poor soil using these practices because the amount of organic material being added is small relative to the mineral proportion of the soil.
Successful composting depends upon the sufficient availability of organic materials, water, manure and “cheap” labour. Where these inputs are guaranteed, composting can be an important method of sustainable and productive agriculture. It has ameliorative effects on soil fertility and physical, chemical and biological soil properties. Well-made compost contains all the nutrients needed by plants. It can be used to maintain and improve soil fertility as well as to regenerate degraded soil. However, materials for compost production may be in short supply and the technology demands high labour inputs for proper compost production and application. Therefore, compost application may be restricted to certain crops and limited application areas, e.g. vegetable production in home gardens.
Mulch or permanent soil cover
One way to improve the condition of the soil is to mulch the area requiring amelioration. Mulches are materials placed on the soil surface to protect it against raindrop impact and erosion, and to enhance its fertility (FAO, 1995). Crop residue mulching is a system of maintaining a protective cover of vegetative residues such as straw, maize stalks, palm fronds and stubble on the soil surface (Plate 17).
The system is particularly valuable where a satisfactory plant cover cannot be established rapidly when erosion risk is greatest (FAO, 1993).
Mulching adds organic matter to the soil, reduces weed growth, and virtually eliminates erosion during the period when the ground is covered with mulch.
There are two principal mulching systems:
- in situ mulching systems – plant residues remain where they fall on the ground (Plate 17);
- cut-and-carry mulching systems – plant residues are brought from elsewhere and used as mulch (Plates 18 and 19).
Crop residue mulching has numerous positive effects on crop production. However, it may require a change in existing cropping practices. For example, farmers may conventionally burn crop residues instead of returning them to the soil. In situ mulching depends on the design of appropriate cropping systems and crop rotations, which have to be integrated with the farming system. The greater labour demands of cut-and-carry systems represent a major constraint. Mulch may be more relevant in home gardens or for valuable horticulture crops (Box 7) than in less intensive farming systems.
Plate 17 No-till maize under black oat mulch, Santa Catarina, Brazil
- BENITES
Mulch affects the soil life. Holland and Coleman (1987) have demonstrated that litter placement on the soil surface (as opposed to incorporation with ploughing) increased the ratio of fungi to bacteria – the reason being that fungi have a higher carbon assimilation efficiency than bacteria. In addition, it encourages bioturbating (mixing) effects of macrofauna that pull the materials into surface layers of the soil.
Decreased decomposition rates
Reduced or zero tillage
Repetitive tillage degrades the soil structure and its potential to hold moisture, reduces the amount of organic matter in the soil, breaks up aggregates, and reduces the population of soil fauna such as earthworms that contribute to nutrient cycling and soil structure.
Avoiding mechanical soil disturbance implies growing crops without mechanical seedbed preparation or soil disturbance since the harvest of the previous crop. The term zero tillage is used for this practice synonymously with terms such as no-till farming, no tillage, direct drilling, and direct seeding.
Compared with conventional tillage, reduced or zero tillage has two advantages with respect to soil organic matter. Conventional tillage stimulates the heterotrophic microbiological activity through soil aeration, resulting in increased mineralization rate. Through breakdown of soil structure, it decreases upward and downward movements of soil fauna, such as earthworms, which are largely responsible for “humus” production through the ingestion of fresh residues. Reduced or zero tillage regulates heterotrophic microbiological activity because the pore atmosphere is richer in CO2/O2, and facilitates the activity of the “humifiers” (Pieri et al., 2002).
Plate 18 Farmer at work in a field of lettuces.
- BIZARRI/FAO/18678
BOX 7 Mulching in the highlands of northern ThailandWhy certain crops receive mulch and others do not Mulching provides a particular benefit to the cultivated crop. Mulching is practised for various cash crops for specific reasons. Onion and garlic are mulched mainly to control weeds (early hand weeding would be difficult without damaging the crop). The mulch is also important to keep the soil moist and cool as these crops are usually grown during the dry season under irrigation. Mulch is also applied under flowers and strawberries, mainly to protect the fragile and valuable products from becoming soiled.Mulching saves labour. Mulching is often seen in maize fields, before as well as after crop establishment. Maize can compete reasonably well with weeds. Therefore, some farmers plant maize without tillage in a mulch of weeds previously killed with herbicides – a system that is less labour-demanding than a tillage operation. Because maize is planted with large spacings, it generally requires less rigorous weeding. Weeding is often done by slashing, and the weed residues are left on the ground. Plate 19 Mulching in the highlands of Northern Thailand.FAO(Source: Van Keer et al., 1996) |
Tillage has become the most common method to control weeds. However, mulching is a more environmentally sound practice than tillage for weed control. The loose soil that results from tilling has less structure than before; the appearance is deceptive. Subsequent traffic or heavy rain soon packs this loosened soil, not only negating the expensive cultivation that produced the loose soil but also culminating in a degraded environment for water entry, seed germination and root growth. Further cultivation is then required to re-loosen the soil; more expense with the same outcome – subsequent repacking and degraded soil structure. This is a typical “downward spiral” of conventional agriculture. Moreover, tillage when the soil is too moist or too dry leads to compaction or pulverization of soil; but farmers may not have the option to wait for optimal conditions.
Severe, accelerated soil erosion and the high costs in terms of labour and energy associated with plough-based methods of seedbed preparation have led to the widespread adoption of no- or zero-tillage systems for cropping in temperate and tropical climates. In no-tillage systems, the crop is sown into a soil left undisturbed since the harvest of the previous crop. Crop residue mulch is maintained and anchored firmly to the ground. Weed control relies on mechanical slashing or cover crops (FAO, 1993). Contact herbicides are also used in some cases.
In reduced- or zero-tillage systems, soil fauna resume their bioturbating activities gradually. These loosen the soil and mix the soil components (also known as biotillage). The additional benefit of the increased soil organic matter and burrowing is the creation of a stable and porous soil structure without expensive, time-consuming and potentially degrading cultivations.
In zero-tillage systems, the action of soil macrofauna gradually incorporate cover crop and weed residues from the soil surface down into the soil. The activity of microorganisms is also regulated by the activity of the macrofauna, which provide them with food and air through their burrows. In this way, nutrients are released slowly and can provide the following crop with nutrients.
Several authors have demonstrated that some crop rotations and zero tillage favour Bradyrhizobia populations, nodulation and thus N fixation and yield (Voss and Sidirias, 1985, Hungria et al., 1997, Ferreira et al., 2000). Figure 10 indicates a 200-300-percent increase in population size of root nodule bacteria in a zero-tillage system compared with conventional tillage. The presence of soybean in the crop rotation resulted in a fivefold to tenfold increase in population size of the same bacteria compared with cropping systems without soybean.
| FIGURE 10 Population size of root nodule bacteria with different crop rotations |
Note: S = soybean; W = wheat; M = maize. Source: Voss and Sidirias, 1985.
Strictly speaking, the term zero tillage applies to methods involving no soil disturbance whatsoever, a condition that may be difficult to achieve. Broadcasting of seed is one way of applying zero tillage (Plate 20). The seed is broadcast over the previous crop residues and, where necessary, the residues are shaken to ensure that the seed falls on the soil surface.
In direct drilling, seeds such as maize, sorghum, soybean, wheat and barley are sown directly into shallow furrows cut into the previous crop residues (Plate 21). Weeds are controlled mechanically with a knife, which knocks down the plants and breaks their stems, or chemically with herbicides.
Traditional practices such as the burning of crop residues may inhibit the introduction of no-tillage systems. In many situations, a conflict exists between leaving crop residues on the surface or feeding them to livestock in the dry season when there is a shortage of fodder.
Mechanical soil disturbance also includes soil compaction through wheel impact of machinery, especially important in large-scale mechanized agriculture, e.g. plantations (sugar cane) or biannual crops (cotton). In a zero-tillage farming system, consideration must be given to reducing both the random placement of tyres/wheels in fields as well as the potential for compaction from animal hooves. Pietola, Horn and Yli-Halla (2003) reported the destructive effect of cattle trampling on the soil structure. Proffitt, Bendotti and McGarry (1995) demonstrated the almost total loss of soil porosity in the soil surface as a result of trampling by sheep. There is a belief that draught animals cause less land degradation than tractors. However, there are reports of soil compaction on smallholder farming enterprises in both Malawi (Douglas et al., 1999) and Bangladesh (Brammer, 2000). The hooves of draught animals and the shearing effect of ploughs or hand hoes, which are used repeatedly at a constant depth, can cause severe compacted layers. Grazing animals should be removed from zero-till fields in moist-wet soil conditions as the compaction risk is greatest at these times.
Plate 20 “Frijol tapado” or broadcast beans on the residues of maize; a common practice in Latin America.
A.J. BOT
![a0100e0s[1]](http://www.econatics.co.za/wp-content/uploads/2015/01/a0100e0s1.jpg)
Plate 21 Maize seedling directly drilled in residues of wheat.
- VANEPH
Plate 22 A sign on a farmgate in Australia. The farmer has achieved a working combination of zero tillage and controlled traffic and is reaping the benefits of excellent water holding capacity in the soil, improved organic matter status, more guaranteed harvests, and a more predictable and rewarding farming system.
- MCGARRY
Controlled traffic, where the wheels of all in-field equipment follow permanent, defined tracks, ensures that compaction is restricted to specific known areas (Plate 22). Alternatively, flotation tyres (low ground-pressure tyres) should be fitted to all large tractors, harvesters, in-field grain bins, etc. in order to reduce their compacting potential.
Recent research has demonstrated the devastating effects of compaction from wheel impact on the occurrence and survival of eartworms (Pangnakorn et al., 2003). Earthworm incidence was greater under controlled traffic than under wheeled traffic. Figure 11 shows the immediate effects of wheeling and tillage on the earthworm population. It appears that wheeling has the most detrimental effect on earthworm survival and that where wheeling is followed by tillage the survival rate is much greater. This may indicate that earthworms are able to survive an initial compaction in the field as long as it is relieved immediately. Where it is not, earthworms are inmobilized and unable to find air and nutrients.
| FIGURE 11 Live and dead earthworm numbers per square metre at 0-15 cm of soil depth sampled immediately after treatment |
Source: Pangnakorn et al., 2003.
Chapter 5. Creating drought-resistant soil
http://www.fao.org/docrep/009/a0100e/a0100e08.htm#TopOfPage
Effect of soil organic matter on soil properties
Organic matter affects both the chemical and physical properties of the soil and its overall health. Properties influenced by organic matter include: soil structure; moisture holding capacity; diversity and activity of soil organisms, both those that are beneficial and harmful to crop production; and nutrient availability. It also influences the effects of chemical amendments, fertilizers, pesticides and herbicides. This chapter focuses on those properties related to soil moisture and water quality, while Chapter 6 focuses on those related to sustainable food production.
Inefficient use of rainwater
Drylands may have low crop yields not only because rainfall is irregular or insufficient, but also because significant proportions of rainfall, up to 40 percent, may disappear as runoff. This poor utilization of rainfall is partly the result of natural phenomena (relief, slope, rainfall intensity), but also of inadequate land management practices (i.e. burning of crop residues, excessive tillage, eliminating hedges, etc.) that reduce organic matter levels, destroy soil structure, eliminate beneficial soil fauna and do not favour water infiltration. However, water “lost” as runoff for one farmer is not lost for other water users downstream as it is used for recharging groundwater and river flows.
Where rainfall lands on the soil surface, a fraction infiltrates into the soil to replenish the soil water or flows through to recharge the groundwater. Another fraction may run off as overland flow and the remaining fraction evaporates back into the atmosphere directly from unprotected soil surfaces and from plant leaves.
The above-mentioned processes do not occur at the same moment, but some are instantaneous (runoff), taking place during a rainfall event, while others are continuous (evaporation and transpiration).
To minimize the impact of drought, soil needs to capture the rainwater that falls on it, store as much of that water as possible for future plant use, and allow for plant roots to penetrate and proliferate. Problems with or constraints on one or several of these conditions cause soil moisture to be one of the main limiting factors for crop growth.
The capacity of soil to retain and release water depends on a broad range of factors such as soil texture, soil depth, soil architecture (physical structure including pores), organic matter content and biological activity. However, appropriate soil management can improve this capacity.
Practices that increase soil moisture content can be categorized in three groups: (i) those that increase water infiltration; (ii) those that manage soil evaporation; and (iii) those that increase soil moisture storage capacities. All three are related to soil organic matter.
In order to create a drought-resistant soil, it is necessary to understand the most important factors influencing soil moisture.
Increased soil moisture
Organic matter influences the physical conditions of a soil in several ways. Plant residues that cover the soil surface protect the soil from sealing and crusting by raindrop impact, thereby enhancing rainwater infiltration and reducing runoff. Surface infiltration depends on a number of factors including aggregation and stability, pore continuity and stability, the existence of cracks, and the soil surface condition. Increased organic matter contributes indirectly to soil porosity (via increased soil faunal activity). Fresh organic matter stimulates the activity of macrofauna such as earthworms, which create burrows lined with the glue-like secretion from their bodies and are intermittently filled with worm cast material.
The proportion of rainwater that infiltrates into the soil depends on the amount of soil cover provided (Figure 12). The figure shows that on bare soils (cover = 0 tonnes/ha) runoff and thus soil erosion is greater than when the soil is protected with mulch. Crop residues left on the soil surface lead to improved soil aggregation and porosity, and an increase in the number of macropores, and thus to greater infiltration rates.
Increased levels of organic matter and associated soil fauna lead to greater pore space with the immediate result that water infiltrates more readily and can be held in the soil (Roth, 1985). The improved pore space is a consequence of the bioturbating activities of earthworms and other macro-organisms and channels left in the soil by decayed plant roots.
On a site in southern Brazil, rainwater infiltration increased from 20 mm/h under conventional tillage to 45 mm/h under no tillage (Calegari, Darolt and Ferro, 1998). Over a long period, improved organic matter promoted good soil structure and macroporosity. Water infiltrates easily, similar to forest soils (Figure 13).
The consequence of increased water infiltration combined with a higher organic matter content is increased soil storage of water (Figure 14). Organic matter contributes to the stability of soil aggregates and pores through the bonding or adhesion properties of organic materials, such as bacterial waste products, organic gels, fungal hyphae and worm secretions and casts. Moreover, organic matter intimately mixed with mineral soil materials has a considerable influence in increasing moisture holding capacity. Especially in the topsoil, where the organic matter content is greater, more water can be stored.
| FIGURE 12 Effect of amount of soil cover on rainwater runoff and infiltration |
Source: Ruedell, 1994.
| FIGURE 13 Water infiltration under different types of management |
Source: Machado, 1976.
| FIGURE 14 Quantity of water stored in the soil under conventional tillage and conservation agriculture |
Source: Gassen and Gassen, 1996.
The quality of the crop residues, in particular their chemical composition, determines the effect on soil structure and aggregation. Blair et al. (2003) report a rapid breakdown of medic (Medicago truncatula) and rice (Oryza sativa) straw residues resulting in a rapid increase in soil aggregate stability through the release of many soilbinding components. As these compounds undergo further breakdown, they will be lost from the system resulting in a decline in soil aggregate stability over time. The slow release of soil-binding agents from flemingia (Flemingia macrophylla) residues resulted in a slower but more sustained increase in the stability of soil aggregates. This indicates that continual release of soil-binding compounds from plant residues is necessary for continual increases in soil aggregate stability to occur.
| FIGURE 15 Effect of different soil covers on in-soil storage of water |
Source: Siqueira et al., 1993.
Elliot and Lynch (1984) showed that soil aggregation is caused primarily by polysaccharide production in situations where residues have a low N content. There is a strong relationship between soil carbon content and aggregate size. An increase in soil carbon content led to a 134-percent increase in aggregates of more than 2 mm and a 38-percent decrease in aggregates of less than 0.25 mm (Castro Filho, Muzilli and Podanoschi, 1998). The active fraction of soil C (Whitbread, Lefroy and Blair, 1998) is the primary factor controlling aggregate breakdown (Bell et al., 1999).
In addition, although they do not live long and new ones replace them annually, the hyphae of actinomycetes and fungi play an important role in connecting soil particles (Castro Filho, Muzilli and Podanoschi, 1998). Gupta and Germida (1988) showed a reduction in soil macroaggregates correlated strongly with a decline in fungal hyphae after six years of continuous cultivation.
The in-soil storage of water depends not only on the type of land preparation but also on the type of cover or previous vegetation on the soil. Figure 15 indicates the effect of burning vegetation on the amount of water stored in the soil.
Conserving fallow vegetation as a cover on the soil surface, and thus reducing evaporation, results in 4 percent more water in the soil. This is roughly equivalent to 8 mm of additional rainfall. This amount of extra water can make the difference between wilting and survival of a crop during temporary dry periods.
A study conducted in 1999 in Guatemala, Honduras and Nicaragua to evaluate the resilience of agro-ecosystems showed that 3-15 percent more water was stored in the soil under more ecologically sound practices (Table 4).
Unger (1978) showed that high wheat-residue levels resulted in increased storage of fallow precipitation, which subsequently produced higher sorghum grain yields. High residue levels of 8-12 tonnes/ha resulted in about 80-90 mm more stored soil water at planting and about 2.0 tonnes/ha more of sorghum grain yield compared to no residue management.
TABLE 4 Average soil depth at which moisture starts, and difference in moisture stored
| Country | Agro-ecologically sound practices cm | Conventional practices cm | Difference (%) |
| Honduras | 9.98 | 10.28 | 2.9 |
| Guatemala | 2.44 | 2.99 | 15.0 |
| Nicaragua | 15.81 | 17.80 | 11.2 |
Source: World Neighbors, 2000.
The addition of organic matter to the soil usually increases the water holding capacity of the soil. This is because the addition of organic matter increases the number of micropores and macropores in the soil either by “gluing” soil particles together or by creating favourable living conditions for soil organisms. Certain types of soil organic matter can hold up to 20 times their weight in water (Reicosky, 2005). Hudson (1994) showed that for each 1-percent increase in soil organic matter, the available water holding capacity in the soil increased by 3.7 percent. Soil water is held by adhesive and cohesive forces within the soil and an increase in pore space will lead to an increase in water holding capacity of the soil. As a consequence, less irrigation water is needed to irrigate the same crop (Table 5).
TABLE 5 Economy of irrigation water through soil cover, the Brazilian Cerrados
| Country | Agro ecologically sound practices (cm) | Conventional practices | Difference (%) |
| Honduras | 9.98 | 10.28 | 2.9 |
| Guatemala | 2.44 | 2.99 | 15.0 |
| Nicaragua | 15.81 | 17.80 | 11.2 |
Source: Pereira, personal communication, 2001.
Reduced soil erosion and improved water quality
The less the soil is covered with vegetation, mulches, crop residues, etc., the more the soil is exposed to the impact of raindrops. When a raindrop hits bare soil, the energy of the velocity detaches individual soil particles from soil clods. These particles can clog surface pores and form many thin, rather impermeable layers of sediment at the surface, referred to as surface crusts. They can range from a few millimetres to 1 cm or more; and they are usually made up of sandy or silty particles. These surface crusts hinder the passage of rainwater into the profile, with the consequence that runoff increases. This breaking down of soil aggregates by raindrops into smaller particles depends on the stability of the aggregates, which largely depends on the organic matter content.
Increased soil cover can result in reduced soil erosion rates close to the regeneration rate of the soil or even lower, as reported by Debarba and Amado (1997) for an oats and vetch/maize cropping system (Figure 16).
Soil erosion fills surface water reservoirs with sediment, reducing their water storage capacity. Sedimentation also reduces the buffering and filtering capacity of wetlands and the flood-control capacity of floodplains. Sediment in surface water increases wear and tear in hydroelectric installations and pumps, resulting in greater maintenance costs and more frequent replacement of turbines. Sediments can also reach the sea (Plate 23), harming fish, shellfish and coral. Eroded soil contains fertilizers, pesticides and herbicides; all sources of potentially harmful off-site impacts.
When the soil is protected with mulch, more water infiltrates into the soil rather than running off the surface. This causes streams to be fed more by subsurface flow rather than by surface runoff. The consequence is that the surface water is cleaner and resembles groundwater more closely compared with areas where erosion and runoff predominate. Greater infiltration should reduce flooding by increased water storage in soil and slow release to streams. Increased infiltration also improves groundwater recharge, thus increasing well supplies.
| FIGURE 16 Soil loss due to water erosion for different maize cropping systems |
Note: Corrected with soil regeneration = 1.7 tonnes/ha/year. Source: Debarba and Amado, 1997.
Bassi (2000) reported significant reductions in water turbidity and sediment concentration over a period of ten years (1988-1997) in different catchment areas in southern Brazil. The reductions varied between 50 and 80 percent depending on locally predominant soil types. These reductions were caused by increases in the incidence of planting perennial crops (banana and pasture) on hillsides, thereby decreasing erosion potential. Total sediment loss decreased by 16 percent and the cost of fertilizers declined by 21 percent; an indication of the previous loss of fertilizers with the eroded soil. Guimarães, Buaski and Masquieto (2005) illustrate the same effect for one specific catchment. The catchment area of Rio do Campo, Paraná, provides 80 percent of the water supply for Campo Mourão, a city with an urban population of 357 000. In the period 1982-1999, a drastic reduction in water turbidity was measured (Figure 17).
Sediment and dissolved organic matter in surface water have to be removed from drinking-water supplies. Reduced erosion, and hence fewer soil particles in suspension, lead to lower costs for water treatment. Data from Chapecó, Brazil, indicate that the quantity of aluminium sulphate used for flocculating suspended solids fell by 46 percent in five years. Where water is chlorinated to kill disease organisms, the chlorine reacts with dissolved organic matter to form trihalomethane (THM) compounds such as chloroform. THMs are suspected of causing cancers (Fawcett, 1997). Reductions in runoff and erosion should lead to reduced formation of THMs during the chlorination process.
Erosion may also have long-lasting secondary consequences through effects on plant growth and litter input (Gregorich et al., 1998). If erosion suppresses productivity, thereby limiting replenishment of organic matter, the amount of organic matter may spiral downwards in the long term.
Soil cover protects the soil against the impact of raindrops, prevents the loss of water from the soil through evaporation, and also protects the soil from the heating effect of the sun. Soil temperature influences the absorption of water and nutrients by plants, seed germination and root development, as well as soil microbial activity and crusting and hardening of the soil.
Roots absorb more water at higher soil temperatures up to a maximum of 35 °C. Higher temperatures restrict water absorption. Soil temperatures that are too high are a major constraint on crop production in many parts of the tropics. Maximum temperatures exceeding 40 °C at 5 cm depth and 50 °C at 1 cm depth are commonly observed in tilled soil during the growing season, sometimes with extremes of up to 70 °C. Such high temperatures have an adverse effect not only on seedling establishment and crop growth but also on the growth and development of the micro-organism population. The ideal rootzone temperature for germination and seedling growth ranges from 25 to 35 °C. Experiments have shown that temperatures exceeding 35 °C reduce the development of maize seedlings drastically and that temperatures exceeding 40 °C can reduce germination of soybean seed to almost nil.
Mulching with crop residues or cover crops regulates soil temperature. The soil cover reflects a large part of solar energy back into the atmosphere, and thus reduces the temperature of the soil surface. This results in a lower maximum soil temperature in mulched compared with unmulched soil (Figure 18) and in reduced fluctuations.
Plate 23 Runoff and soil loss immediately after a rainstorm, Naisi catchment. Zomba Mountain, Malawi.
T.F. SHAXSON
| FIGURE 17 Development of water turbidity rates in the catchment area of Rio do Campo, Paraná, 1982-1999 |
Source: Guimarães, Buaski and Masquieto, 2005.
| FIGURE 18 Temperature fluctuations at a soil depth of 3 cm in a cotton crop with and without a soil cover of mucuna |
Source: Derpsch, 1993.
Chapter 6. Key factors in sustained food production
http://www.fao.org/docrep/009/a0100e/a0100e09.htm#TopOfPage
Increased plant productivity
Plant productivity is linked closely to organic matter (Bauer and Black, 1994). Consequently, landscapes with variable organic matter usually show variations in productivity. Plants growing in well-aerated soils are less stressed by drought or excess water. In soils with less compaction, plant roots can penetrate and flourish more readily. High organic matter increases productivity and, in turn, high productivity increases organic matter.
Increased fertilizer efficiency
The two major soil fertility constraints of the West African savannah and in the subhumid and semi-arid regions of SSA are low inherent nutrient reserve and rapid acidification under continuous cultivation as a consequence of low buffering or cation exchange capacity (Jones and Wild, 1975). Generally, these constraints are tackled by applying chemical fertilizers and lime. However, the application of inorganic fertilizers on depleted soils often fails to provide the expected benefits. This is basically because of low organic matter and low biological activity in the soil.
The chemical and nutritional benefits of organic matter are related to the cycling of plant nutrients and the ability of the soil to supply nutrients for plant growth. Organic matter retains plant nutrients and prevents them leaching to deeper soil layers. Microorganisms are responsible for the mineralization and immobilization of N, P and S through the decomposition of organic matter (Duxbury, Smith and Doran, 1989). Thus, they contribute to the gradual and continuous liberation of plant nutrients. Available nutrients that are not taken up by the plants are retained by soil organisms. In organic-matter depleted soils, these nutrients would be lost from the system through leaching and runoff.
Phosphate fixation and unavailability is a major soil fertility constraint in acid soils containing large amounts of free iron and aluminium oxides. In comparing the P-sorption capacity of surface and subsurface soil samples, Uehara and Gilman (1981) provided indirect evidence that soil organic matter can reduce the P-sorption capacity of such soils. This implies that for high P-fixing soils, i.e. oxide-rich soils derived from volcanic and ferro-magnesian rocks, management systems that are capable of accumulating and maintaining greater amounts of calcium-saturated soil organic matter in the surface horizon would increase P availability from both organic and fertilizer sources.
Weak acids, such as the organic acids in humus, do not relinquish their hydrogen (H) easily. H is part of the humus carboxyl (-COOH) under acidic conditions. When a soil is limed and the acidity decreases, there is a greater tendency for the H+ to be removed from humic acids and to react with hydroxyl (OH-) to form water. The carboxyl groups on the humus develop negative charge as the positively charged H is removed. When the pH of a soil is increased, the release of H from carboxyl groups helps to buffer the increase in pH and at the same time creates the CEC (negative charge). With an increase in organic matter, the soil recovers its natural buffer capacity; this means an increase in pH in acid soils (Figure 19).
| FIGURE 19 Effect of organic matter on soil pH |
Note: Original pH level was 5.3. Source: Mielniczuk, 1996.
TABLE 6 Incidence of lime in the soil profile under different soil covers over the same period after surface application
| Cover | Soil depth (cm) |
| Bare soil | 0-7 |
| Black oats | 0-20 |
| Oil radish | 0-22 |
CEC is linked closely to the organic matter content of the soil. It increases gradually with time where organic residues are retained, first in the topsoil and later also at greater depth. Crovetto (1997) reported an increase in CEC of 136 percent (from 11 to 26 meq/100 g of soil) as a consequence of humus increase in the topsoil after 20 years of residue retention.
To overcome acidity, lime is usually incorporated in the soil. However, organic matter on the soil surface favours the transport of calcium carbonate (lime) to deeper soil layers after surface application (Table 6).
The crop residues release organic acids that cause the lime to penetrate deeper into the profile much more rapidly than when applied on bare soil. Thus, it is no longer necessary to mix lime intensively into the soil, which is appropriate for farming systems based on reduced or zero tillage.
Reduced waterlogging
Chapter 5 examined the water storage capacity of soils under improved organic management. However, in case of waterlogging, organic matter plays also an important role. The bioturbating activity of the macrofauna leaves various so-called conducting macropores in the soil, which are responsible for the drainage of water to deeper soil layers.
Chan et al. (2003) found a significant reduction in waterlogging after three years under no tillage compared with conventional tillage. The reduction was related to higher density of conducting macropores (140/m2 and 5/m2 for no tillage and conventional tillage, respectively), which was associated with higher population density of earthworms (240/m2 and 36/m2 for no tillage and conventional tillage, respectively). Plate 24 provide a demonstrationof this effect.
Increased yields
Agronomic practices that influence nutrient cycling, especially mineralization and immobilization, result in an immediate productivity gain or loss, which is reflected in the economics of the agricultural system.
Crop yields in systems with high soil organic matter content are less variable than those in soil that are low in organic matter. This is because of the stabilizing effects of favourable conditions of soil properties and microclimate. Improvements in crop growth and vigour stem from direct and indirect effects. Direct effects stem from improvements in nutrient and water content, as described above (also Box 8). Indirect effects stem from a favourable rooting environment and possible weed suppression and a reduction in pests and diseases.
Plate 24 Under no-tillage conditions, the internal pore system of the soil is not distroyed through land preparation activities and able to drain rainwater from the surface to deeper layers (left). On bare soil the impact of raindrops results in sealing of the surface pores and thus poor drainage.
- BOT
| BOX 8 Does improved organic matter management pay?There are several ways to calculate the rentability of a farm. In general, key parameters such as expenses, income, yield, cost of production and gross margin are used to analyse how well farmers have managed to reach an income. However, in areas where water is a limiting factor, it may be more useful to analyse the conversion of water into yield or even money. The following example illustrates different ways to compare outcomes of farms and their cropping systems. It is based on the cropping results of 16 farms with a cropped surface of 60 000 ha in Australia. One-third of this surface is cropped in a conventional way (CT), using intensive tillage practices, fire and several passages with herbicides for weed control. Two-thirds of the area is cropped with the aim to retain as much ground cover as possible, and thus improve the organic matter content of the soil, using specialized planters and herbicides to control weeds, also known as conservation farming (MT). An analysis of the cropping system data for three years yielded the following gross margins for three crops in rotation (wheat, chickpea and sorghum):
However, when using water-use efficiency (WUE) as means of comparing outputs across farming systems, the results are even more drastic. In this case, rather than the common method of determining WUE by breaking the season into fallow and in-crop components, a total water-use efficiency factor was used. Both grain yield and gross margin are divided by total water to obtain an insight into how well farmers managed the conversion of water into yield and money in the year 1997.Source: Rummery and Coleman, 1999. |
Immobilization of N may occur in systems with crop residue management, especially where C:N ratios of the residues are high (tough, woody materials). This can cause a decrease in maize yield. Figure 20 shows the effect of tillage and the preservation of crop residues on maize yield. The preservation of wheat and horse-radish residues on the soil surface led to immobilization of N, which was overcome through the application of N fertilizer. Based on these data, maize with oats, lupine and vetch as a winter cover crop (without fertilization) can produce a yield that is comparable with or higher than those obtained with conventional tillage and a fertilizer treatment of 90 kg/ha. In this case, the yield increase was highly correlated with the P content of the leaves and the P availability in the soil. This occurred because of the higher moisture content in the soil under the mulch layer, which led to a higher P uptake by plant roots.
Grain crops can also have residual effects on each other through the decomposition of chemical compounds in the residues.
Reduced herbicide and pesticide use
Some people are concerned that intensified systems with reduced or zero tillage will increase herbicide use and in turn lead to increased contamination of water by herbicides. According to Fawcett (1997), total herbicide use in the United States of America declined during the period of adoption of no-tillage systems. He concludes that herbicides are important, but that farmers using conventional tillage methods use similar amounts of herbicides to no-tillage farmers. In Honduras, a strong decline in the use of herbicides has been observed (Figure 21). Farmers who no longer burn their fields prior to preparation spend less money on herbicides. Farmers who have adopted the Quezungual system spend less on herbicides and make savings, both in terms of land preparation and total costs.
It is becoming evident that the need for herbicide use diminishes over time in wellmanaged no-tillage cropping systems. The principal reason is that the system reduces the existing seed bank in the soil by the synergy of two activities: reduction of the production of new seeds through avoidance of flowering and fruit setting; and reduction of seeds that are brought to the surface by tillage practices.
| FIGURE 20 Maize yield under conventional tillage and direct sowing, with and without 90 kg of N fertilizer |
Source: Calegari, 1998.
| FIGURE 21 Herbicide costs in different production systems in Lempira Sur, Honduras |
Source: CDR, 200
With direct seeding, the reservoir of seeds differs from conventional tillage because:
- the weed seeds remain on the soil surface, where they are susceptible to attacks from insects, birds and soil organisms and to atmospheric influences;
- the soil remains covered by residues, which prevents light reaching the seeds and thus reduces germination;
- weed seeds already at certain depths are not brought to the surface again, where they could germinate;
- perennial weeds are no longer redistributed by equipment.
The result is that the soil weed seed store diminishes in time and, consequently, the weed problem also diminishes, as does the need to use herbicides. The concentration of soil organic matter in the topsoil layer plays an important role in the absorption of herbicides. When the concentration of organic matter in the topsoil decreases, contamination of the environment by herbicides is likely to increase. Enhanced levels of organic matter cause enhanced adsorption of pesticides, followed by gradual degradation. Herbicides, like other pesticides, can be used by micro-organisms as a substrate to feed on (Haney, Senseman and Hons, 2002). Herbicides are broken down in soil and water by micro-organisms into natural acids, NH4–, amino acids, carbohydrates, phosphate and CO2 (Schuette, 1998). As microbes cause more rapid degradation of pesticides, enhancing microbial activity may reduce leaching of pesticides.
Many herbicides, including glyphosate and paraquat, which are the most common herbicides used in reduced-tillage systems, are bound tightly to clay and organic matter by electrostatic forces and hydrogen bonding. Once they are bound to soil organic matter, the herbicides become inactive and no longer affect plants. Moreover, they can form insoluble complexes with metals in the soils. This also contributes to their rare stability in the soil and low potential for leaching into groundwaters (Ahrens, 1994a, 1994b). Some studies have shown that there is no reason to believe that glyphosate may cause any unexpected damage to the environment (Torstenson, 1985). However, other studies illustrate negative effects on soil life or its functions.
Farming systems that increase soil organic matter content (e.g. no tillage) reduce the probability of environmental contamination by herbicides.
Increased biodiversity
Conventional agriculture tends to reduce aboveground and belowground diversity. Thus, it brings about significant changes in the vegetation structure, cover and landscape. The change in vegetal cover during the conversion of forest and pastures to cropping affects plants, animals and micro-organisms. Through increasing specialization of certain plant species (food and fibre crops, pasture and fodder crops, and tree crops) and livestock species, some functions may be affected severely, e.g. nutrient cycling and biological control. Some non-harvested or associated species profit from the change and become pests. However, many organisms either disappear completely or their numbers are reduced drastically, e.g. pollinators and beneficial predators, unless efforts are made to retain a suitable habitat (Box 9).
Associated species can be managed to a certain extent. Through appropriate crop rotations, crop-livestock interactions and the conservation of soil cover, a habitat can be created for a number of species that feed on pests. This will in turn attract more insects, birds and other animals. Thus, rotations and associations of crops and cover crops as well as hedgerows and field borders promote biodiversity and ecological functions. Because of the complexity and richness of soil biodiversity, the effects of crop and pasture management are less well understood. However, the effects on certain functional groups and, hence, specific soil functions are being recognized increasingly as vital for agricultural productivity and system sustainability.
| BOX 9 Effect of different tillage practices on scarab beetle-grub holes and their volumesBefore the transformation of native grasslands and forests into agricultural areas, a large number of species of scarab beetles and their larvae (white grubs) inhabitated the soils in southern Brazil. With the transformation of these areas, some of these beetles disappeared, while others became so well adapted and, lacking biological control agents, became important soil-dwelling pests. However, there are other species that can be considered as facultative pests. The larvae prefer feeding on surface litter or surface deposited animal excrements, but may become pests when not enough surface litter is present for them to feed on, like the genera Diloboderus and Bothynus. The large beetles create large, permanent galleries (holes) in the soil, down to a depth of more than 1 m, in which they spend most of their lives. The holes may serve as preferential pathways for water infiltration and root growth, and the chambers become niches of increased soil fertility. Both chambers and galleries provide temporary and permanent refuge for many other soil-dwelling invertebrates and microfauna.Research revealed that beetle grub holes were more abundant under no tillage (NT) (8.8-9.6 holes/ m2) than conventional tillage (CT) (0.7-1.3 holes/m2). The largest and deepest holes were also found in NT (up to 33.5 mm in diameter and 117 cm deep). Consequently, the total volume of pores opened in NT (450-503 cm3) was up to almost 10 times greater than in CT (53-107 cm3).Source: Brown et al., 2003. |
Soil has the ability to restore its life-support processes provided that the disturbance is not too drastic and that sufficient time is allowed for such recovery. Organic matter and biodiversity of soil organisms are the driving factors in this restoration. Decreases in numbers and types of soil organisms and available substrate (organic matter) lead to a decrease in resilience, which in turn can result in a downward spiral of degradation.
Resilience
Resilience can be defined as the ability of a system to recover after disturbance (Elliot and Lynch, 1994). Soil resilience depends on a balance between restorative and degrading processes. Factors affecting resilience can be grouped in two categories: endogenous and exogenous. Endogenous factors are related to inherent soil properties (rooting depth, texture, structure, topography and drainage) and microclimate and mesoclimate. Exogenous factors include land use and farming system, technological innovations and input management (Lal, 1994). Hence, appropriate agricultural practices can influence these factors in order to enhance soil resilience.
Organic matter and soil organisms play important roles in conserving and improving soil properties that are related to soil resilience. In addition to creating more pores through biological activity, organic matter plays an important role in the formation and stabilization of soil aggregates through bonds between the organic matter and the mineral soil particles. Soil aggregation can take place through two binding agents:
- waste products of bacteria – polysaccharides;
- fungal and bacterial hyphae.
The preservation of aggregate stability is important in order to reduce surface sealing and increase water infiltration rates (Whitbread, Lefroy and Blair, 1998). With increased stability, surface runoff is reduced (Roth, 1985).
Chapter 7. The role of conservation agriculture in organic matter deposition and carbon sequestration
http://www.fao.org/docrep/009/a0100e/a0100e0a.htm#TopOfPage
Principles of conservation agriculture
Conservation agriculture makes use of soil biological activity and cropping systems to reduce the excessive disturbance of the soil and to maintain the crop residues on the soil surface in order to minimize damage to the environment and provide organic matter and nutrients. It is based on four principles:
- minimal mechanical soil disturbance, mainly through direct seeding;
- permanent soil cover, organic matter supply through the preservation of crop residues and cover crops;
- crop rotation for biocontrol and efficient use of the soil profile;
- minimal soil compaction.
Although the principles are not new (except for that of minimal disturbance to the soil), it is the fact that they are applied together in conservation agriculture that generates positive outcomes. All the practices (minimal tillage, soil cover and crop rotation) are combined for synergy and added value. In the past, farmers may have tried but abandoned the use of cover crops or zero tillage because of weed problems or yield declines. There is also a need for improved weed control and rotations for biocontrol of pests and diseases and nutrient uptake. Integration of the conservation agriculture principles provides a win-win situation for both people and the environment, which has catalyzed successful expansion of the area under conservation agriculture worldwide.
Conservation agriculture aims to:
- provide and maintain optimal conditions in the rootzone (maximum possible depth for crop roots) in order to enable them to grow and function effectively and without hindrance in capturing plant nutrients and water;
- ensure that water enters the soil so that: (i) plants have sufficient water to express their potential growth; and (ii) excess water passes through soil to groundwater and streamflow, not over the surface as runoff where it can cause erosion. There is greater potential for increased cropping efficiency as more water is held in the soil profile than under conventional systems;
- increase beneficial biological activity in the soil in order to: (i) maintain and rebuild soil architecture for enhanced water entry and distribution within the soil profile; (ii) compete with potential soil pathogens; (iii) contribute to decomposition of organic materials to soil organic matter and various grades of humus; and (iv) contribute to the capture, retention and gradual release of plant nutrients;
- avoid physical or chemical damage to roots and soil organisms that would disrupt their effective functioning.
Organic matter deposition
The reduction of soil disturbance through zero-tillage, the use of cover crops and the preservation of crop residues on the soil surface result in increased activity of the soil and in the accumulation of organic matter, mainly in the topsoil (Figure 22).
An argument often heard in the discussion on conservation agriculture is that it is only feasible in the humid and subhumid tropics and that the generation of sufficient biomass in semi-arid regions is the limiting factor to start implementing conservation agriculture.
However, recent research has shown that even in semi-arid areas of Morocco the application of the principles of conservation agriculture bears its fruits. Mrabet (2000) reports higher yields through better water use and improved soil quality; the latter caused by an increase in soil organic C and N and a slight pH decline in the seedzone (Bessam and Mrabet, 2003; Mrabet et al., 2001a, 2001b).
Increased carbon sequestration
World soils are important reservoirs of active C and play a major role in the global carbon cycle. As such, soil can be either a source or sink for atmospheric CO2 depending on land use and the management of soil and vegetation (Lal, 2005) (Figure 23). The conversion of native ecosystems (e.g. forests, grasslands and wetlands) to agricultural uses, and the continuous harvesting of plant materials, has led to significant losses of plant biomass and C (Davidson and Ackerman, 1993), thereby increasing the CO2 level in the atmosphere.
In particular, the practice of burning agricultural fields before cultivation has a disastrous effect on soil organic carbon content. Figure 24 shows the reduction in soil organic carbon in agricultural fields after 100 years of burning crop residues and weeds compared with an area that was not burned or ploughed during the same period. The topsoil layer (0-5 cm) represented the greatest carbon loss (36 percent) compared with the area that was not burned. Soil N stock in the same layer was reduced by 16 percent. The carbon stock was reduced not only through burning, but because of the whole land-use management, especially a drastic reduction in diversity of species as monocropping was practised (Amado et al., 2005).
| FIGURE 22 Organic matter content of a soil under different tillage management |
Source: FAO, 2002.
| FIGURE 23 Soil carbon concentration at various soil depths affected by management system |
Note:Conventional (CT) and conservation (NT) agriculture, after two complete cropping cycles (4 years). Source: Prior et al., 2003.
| FIGURE 24 Effect of crop residue burning once every two years on soil carbon stock |
Source: Spagnollo, 2004.
Table 7 lists general practices that determine whether soil will be a sink or a source of atmospheric CO2.
As shown in Table 7, soil can play a part in mitigating CO2 levels (Paustian, 2002). This removal process is achieved naturally, and quite effectively, through photosynthesis. Living plants take CO2 from the air in the presence of sunlight and water, convert it into seeds, leaves, stems and roots. Part of the CO2 is retained or “sequestered”, or stored as C in the soil when decomposed.
In particular, systems based on high crop-residue addition and no tillage tend to accumulate more C in the soil than is lost to the atmosphere. Carbon sequestration in managed soils occurs when there is a net removal of atmospheric CO2 because C inputs (crop residues, litter, etc.) exceed C outputs (harvested materials, soil respiration, C emissions from fuel and the manufacture of fertilizers, etc.) (Izaurralde and Cerri, 2002). Management practices that increase soil C comply with a number of principles of sustainable agriculture: reduced tillage, erosion control, diversified cropping system, balanced fertilization, etc.
In the early years of no-tillage systems, the organic matter content of the soil is increased through the decomposition of roots and the contribution of vegetative residues on the surface. This organic material decomposes slowly, and thus the liberation of C to the atmosphere also occurs slowly. In the total balance, net fixation or sequestration of C takes place; the soil is a net sink of C.
TABLE 7 Land use and land management determining whether soil will be a sink or source of atmospheric CO2
| Soil as a source of CO2 | Soil as a sink of CO2 |
| Soil properties: coarse textured soil, excessive drainage, high susceptibility to erosion | Soil properties: clayey soil, poorly drained ecosystems, depositional sites, including footslopes |
| Land use: seasonal crops, simple ecosystem, shallow roots and low root-shoot ratio | Land use: perrenial crops, diverse ecosystem, deep roots and high root-shoot ratio |
| Soil management: intensive tillage based on plough, negative nutrient balance, residue removal and/or burning, continuous cropping, loss of soil and water by runoff and erosion | Soil management: no tillage, positive nutrient balance, mulch farming cover crops in rotation, cycle, soil and water conservation |
Source: adapted from Lal, 2005.
Figure 25 illustrates the fact that some cropping systems can act as a sink for CO2. In this example, the carbon stock in soils under natural vegetation is used as a reference (steady state: DC = 0). In eight years, the fallow/maize system liberated 4.3 tonnes of CO2 per hectare. The maize/mucuna system showed a positive balance of almost 20 tonnes of CO2 per hectare compared with fallow/maize. Compared with soils under natural vegetation, this means a capture of atmospheric CO2 of more than 15 tonnes/ha in eight years. Lovato (2001) found an increase of 2 tonnes/ha/year over 13 years in a rotation system of oats – common vetch/maiz – cowpea. These figures confirm the potential of conservation agriculture for carbon sequestration. However, the “simple” change from soil tillage to zero tillage is not enough. According to Lovato et al. (2004), a minimum addition of 4.2 tonnes/ha/year of carbon in vegetative residues in cropping systems and 4.5 tonnes/ha/year in mixed systems of pastures and crops (Nicoloso, Lovato and Lanzanova, 2005) is necessary for maintaining soil organic matter at stable levels. This means that below these values CO2 emission will or can take place and, thus, that for conservation agriculture systems to become successful in promoting carbon sequestration, it is necessary to include crops and pastures in the rotation that add large quantities of biomass.
Even more C can be stored by adding leguminous cover crops to the rotation cycle. This is shown in Figure 26, based on two long-term experiments in Rio Grande do Sul, Brazil. Besides addition of C to the soil, legumes add a substantial quantity of N to the soil, which results in increased biomass production of succeeding crops.
| FIGURE 25 Estimation of emission and sequestration of CO2 (total over 8 years) under different maize production systems with cover crops under direct sowing compared with natural vegetation in southern Brazil |
Source: Amado et al., 2001.
Assuming an average carbon accumulation of 0.5 tonne/ha/year, an area like southern Brazil (Rio Grande do Sul, Santa Catarina and Paraná) under conservation agriculture would have the potential to sequester 5 million tonnes of C annually, which corresponds to 18 million tonnes of atmospheric CO2. To compare, Brazil as emitted an estimated 84 million tonnes of CO2 in 2000 (Carbon Dioxide Information Analysis Center, 2003).
Recent studies have shown that soil temperature is one of the main climate factors that influence CO2 emission. High soil temperatures accelerate soil respiration and thus increase CO2 emission (Brito et al., 2005). This has implications for the landscape and land use in a certain area. On convex slopes and hilltops, emission is greater than in foothills, where temperatures are normally lower. The foregoing indicates that not only for soil and water conservation is it important to protect the soil with vegetation (reduction in soil temperature), but that it is advisable to cover the soil also with a view to reducing greenhouse gas emissions.
| FIGURE 26 Potential for carbon sequestration in long-term experiment in southern Brazil |
Notes: CT = conventional tillage, RT = reduced tillage, ZT = zero tillage, CTwN = conventional tillage without N fertilizer. CT oats-maize (=0). Source: Adapted from Amado
Chapter 8. Conclusions
http://www.fao.org/docrep/009/a0100e/a0100e0b.htm#TopOfPage
The maintenance of soil organic matter levels and the optimization of nutrient cycling are essential to the sustained productivity of agricultural systems. Both are related closely to the bioturbating activities of macrofauna and the microbially-driven mobilization and immobilization processes, which the activities of large invertebrates also encourage. Maintaining soil organic matter content requires a balance between addition and decomposition rates. As changes in agricultural practices can engender marked changes in both the pool size and turnover rate of soil organic matter, it is important to analyse their nature and impacts.
Crop production worldwide has generally resulted in a decline in soil organic matter levels and, consequently, in a decline of soil fertility. Converting grasslands and forestlands to arable agriculture results in the loss of about 30 percent of the organic C originally present in the soil profile. On reasonably fertile soils with reliable water supply, yields in long-term arable agricultural systems have been maintained at very high levels by applying substantial amounts of fertilizer and other soil amendments. In low-input agricultural systems, yields generally decline rapidly as nutrient and soils organic matter levels decline. However, restoration is possible through the use of fallow lands, mixed crop-livestock and agroforestry systems, and crop rotations.
Traditional mould-board plough and disc-tillage cropping systems tend to cause rapid decomposition of soil organic matter, leave the soil susceptible to wind and water erosion, and create plough pans below the cultivation depth. By contrast, reduced- or zero-tillage systems leave more biological surface residues, provide environments for enhanced soil activity, and maintain more intact and interconnected large pores and more soil aggregates, which are better able to withstand raindrop impact. Water can infiltrate more readily and rapidly into the soil with reduced tillage and this helps protect the soil from erosion. In addition, organic matter decomposes less rapidly under reduced-tillage systems. No-tillage systems have proved especially useful for maintaining and increasing soil organic matter.
Crop rotation is the basis for the sustainability of direct sowing systems. A production system that includes cover crops, legumes for N fixation, crop rotation and no tillage can be adapted regionally and, therefore, contribute to the sustainability of soil management in the region. Where rainfall intensities are very high, or biomass management options are limited by a water shortage, maintenance of soil surface cover by crop canopies or crop residues during periods of high erosion risk is essential. With improved land management, at least part of the organic matter lost can be restored. The increase in soil organic matter in the absence of tillage can transform agricultural soils into carbon sinks.
The relatively low levels of active organic matter fractions in zero-tillage systems have highlighted the extreme dependence of such systems on the maintenance of high levels of surface protection by crop residues. Residue accumulation, including cover crops and crop residues, increases the levels of some soil nutrients and soil organic C. The active fraction of organic matter plays a very important role in aggregate stability and rainfall infiltration. Building up active C levels in the soil in rainfed cropping systems may have a greater impact in reducing surface crusting and improving rainfall infiltration capacity than would simply changing to zero-tillage systems. Management practices designed to maximize C inputs and to maintain a high proportion of active C should be seen as essential steps towards more sustainable cropping systems.
Cover crops, intercropping and crop rotations can also help to promote biodiversity both below the soil surface and aboveground. This diversity is important to maintaining a well-functioning and stable ecological system. Where many different types of organisms coexist, there are: fewer problems with diseases, insects and nematodes; more competition among species; and more possibility for many types of predators to thrive. In such a situation, no single pest organism is able to reach a population of sufficient size to affect crop yield seriously.
