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The building blocks of soil

The soil that is cultivated to create a seedbed only consists to half of solid material, while the remainder consists of pores filled with water or air. Of the solid material, clay and organic material have the strongest influence on the soil and determine its tillage properties.

Soil consists of around 50% solid material, with 50% pores in between. In simple terms, half of a soil clod is solid material and the other half is pores.

Pores with water or air

The solid material consists of either mineral particles of different size classes or of organic material. The important pores are filled with either air or water depending on how wet the soil is at a particular time, its structure and soil tillage. In the ideal case, half the pores are filled with water and half with air. However, in soils with an aggregated structure such as clays, the pore volume is somewhat higher (40-60%) than in single-grained soils such as sands (35-45%).

Particle size classes

The texture of the soil refers to the distribution of mineral particles into different size classes. Different countries use different classes, but a common internationally acceptable system classifies the texture into blocks, stones, gravel, sand, silt and clay according to the particle size ranges shown in table “Particle size distribution” below.

a) Fine sand b) Very fine sand c) Coarse silt d) Fine silt e) Coarse clay

Figure above shows the difference in size between some of the mineral particles in a soil and the significance of this. Clay and humus particles are the smallest constituents in the soil. Their average diameter is less than 0.0002mm (i.e. 1000 times smaller than a grain of sand) and they are called colloids. The surface of clay particles has a negative electrical charge. This means that positively charged nutrients such as potassium, calcium and magnesium ions can bind to the clay particles. The clay particles therefore comprise nutrient reserves for plants.

Silt holds water

A cultivated arable soil is often a mixture of different particle size groups. If gravel and sand dominate the soil texture, this gives permeable, dry and relatively infertile soils, while an inclusion of sand in a clay soil makes it warmer. Silty soils are often cold and water-retaining and can easily take up water through capillary rise. The finest mineral particles, clay, have a strong influence on the soil even at concentrations of around only 5 %. Clay soils shrink and swell and impart an aggregated stucture to the soil, with cracks and crevices where roots can grown through the soil profile. The typical characteristics of various soils are often a function of the clay content, which strongly influences soil type and tillage.

Organic material positive

The organic material in the soil also has a very clear influence on the character of the soil. It consists to almost 60 % of carbon (C) and originates from plant residues that have been decomposed by microorganisms. In this decomposition process (see image below), plant nutrients such as nitrogen (N), phosphorus (P) and sulphur (S) are released. The organic material can have enormous significance for soil properties and its influence is nearly always positive from the farmers perspective. It affects:

• Structure and aggregate stability
• Water management
• Tillage
• Nutrient reserves
• Silting and crusting

The decomposition process:

a) Soil fauna start decomposition of dead organic material, partly through breaking it down into smaller pieces and partly through digging holes in the soil, increasing the supply of oxygen. Earthworms play a specific and very valuable role by breaking down the material and mixing it into the soil.

b) Bacteria and fungi continue decomposition in stages. The last phase, the formation of simple end-products that are available to plants (e.g. nitrate, phosphate and sulphate), is called mineralisation.

c) Humus formation. The decomposition of various organic compounds proceeds through a series of intermediate products that are of an increasingly simple nature as decomposition advances. These intermediate products react with each other and with the compounds created by soil organisms. This leads to the formation of new chemical compounds that are transformed into high molecular weight, dark-coloured matter known as humic substances. These humic substances have the ability to bind positively charged ions of potassium, calcium and magnesium, for example.

Smallest particles have large specific area

Fine clay (<0.0002 mm) and some organic material are colloids and represent the smallest components in the soil. However, they have a large specific area (see table “Particle size distribution” below), i.e. a large surface area in relation to their weight. The specific area increases with decreasing particle size (see image below). The surface of clay particles is negatively charged, so nutrients in the soil that are cations bind to the surface, creating a store of nutrients for the plants.

A characteristic of all clay minerals is their flattened shape. This, together with their extremely small size, means that clay colloids have a very large surface area in relation to their mass – a high specific area. For example, one gram of sand has a combined area of around 1.5-2 cm², which is equivalent to a small stamp. However, one gram of clay can have a combined area of several 100 m² – an average-sized house.

Particle size distribution (table)

Dictionary:

Pore = soil pores are the spaces, channels and cracks in the soil, which are filled with either water or air depending on the actual water content of the soil.

Mineral particles = soil mineral particles are the inorganic smallest constituents, which have been formed at the site through weathering of various minerals and rock types or have been transported there, e.g. by glaciers. The properties of the soil are strongly dependent on soil particle size according to table Particle size distribution

Texture = soil texture refers to the proportions of mineral particles with different average diameter, i.e. the relative proportions of sand, silt and clay in particular according to table Particle size distribution

Capillary = capillary water is water that can rise upwards in the soil within the fine pores through binding of the water molecules in the pores, adhesion, but also through attraction between water molecules, cohesion. Silty soils have high capillarity and combine a large height of capillary rise with a high rate of capillary rise

Colloid = colloids are the finest particles in the soil, with an average diameter of less than 0.0002 mm. The colloids include some organic material and fine clay

Specific area = the combined surface area of the soil particles is expressed as square metres per gram of dry soil and is an important characteristic since it indicates the amount of nutrients the soil can release through weathering and bind to its surfaces

Cations = positively charged ions in the soil, e.g. potassium, calcium and magnesium

Soil fauna = earthworms, woodlice, springtails, centipedes, mites and other animals that open the door for bacteria and fungi through dividing and breaking apart plant residues in their mouth, stomach and intestines

Characteristics of different soils

a. Sand Soils

Sand soils are often dry, nutrient deficient and fast-draining. They have little (or no) ability to transport water from deeper layers through capillary transport. Therefore, tillage of sandy soils in the spring should be kept to a minimum in order to retain moisture in the seedbed. The nutrient- and water-holding capacity of sand soils can be improved through adding organic material.

b. Silt soils, 0-10% clay

These soils differ from sand soils by having a greater tendency to form a crust, which is often very hard. If they are over-tilled, they can become compact and this decreases their ability to infiltrate water in wet periods. In dry conditions they can become hard and difficult to till. However, they are generally easy to till and can store considerable amounts of water. They require good reconsolidation, but tillage in wet conditions should be avoided.

c. Clay soils with 10-25% clay

These soils differ from those described above in that crusting can be very severe. The crust is often so hard that it has to be broken up. With low contents of clay and organic material, aggregate formation is often poor.

d. Clay soils with 25-40% clay

These soils have a good ability to transport water by capillary action from deep layers but the rate is slow, so plant water requirements are not met through capillary water. These soils are darker in colour and soil aggregation is more distinct. Aggregation decreases the risk of crusting. These soils must be tilled at the correct water content in order to be easily cultivated. There is a risk of clodding if conditions are too dry, or of smearing if they are too wet. These soils have a good ability to improve their structure through the action of climate, roots etc.

e. Clay soils with 40% clay

Heavy clays have a very high water-holding capacity, but most of the water is tightly bound and not available to plants. The humus content is often higher than in other mineral soils. They do not form a crust when they dry. These soils have a very good ability to improve their structure through e.g. freezing/thawing and drying/wetting. In cold winters the clay freezes apart and forms a very favourable aggregated structure in the topsoil layer. If the clay dries out without having been frozen, it can become very stiff and difficult to work. In the water-saturated state these soils can be sticky and very impermeable to water. Due to the high clay content, the nutrient content is very high. Heavy clays need a high degree of recompaction around the seed when they are dry, but not when they are damp and plastic. The risk with tilling them in wet conditions is that it leads to soil compaction.

Dictionary:

Clay = clay is the smallest particle group, with an average particle diameter of less than 0.002 mm. See table “Particle size distribution” in chapter The building blocks of soil.

Capillary = capillary water is water that can rise upwards in the soil within the fine pores through binding of the water molecules in the pores, adhesion, but also through attraction between water molecules, cohesion. Silty soils have high capillarity and combine a large height of capillary rise with a high rate of capillary rise.

Soil structure

Soil structure describes the physical configuration of the soil. The sand particles in a sand soil are held together weakly and do not form aggregates, while the clay particles in a clay soil readily form aggregates. These aggregates make clay soil easy to till and improve the transport of air and water.

Soil structure is defined as the 3-D arrangement of primary particles in the soil, i.e. how the primary particles are situated and connected to each other. Determining factors for soil structure include its texture and content of organic material. A distinction is made between soils with a single-grained structure and soils with an aggregated structure.

Sand soils are not held together

A sand soil such as this (see image above) is an example of a single-grained structure. The grains of sand are relatively large and often only held together weakly. Even sand soils with a higher content of colloids fall apart easily when exposed to pressure in the soil. A sand soil with a low clay content often requires deeper cultivation in order to create a good arable soil and growing site. The low clay content gives the sand soil a low internal structure forming capacity.

Clay soils become aggregated

A clay soil such as this (see image above) is held together and often has an aggregated structure. Even at a content of 5%, the clay has a very strong impact on the soil and dominates its properties.

The aggregated structure is the result of a range of processes in the soil that in combination form aggregates. These structure forming processes affect the soil structure in a dynamic interplay with soil tillage.

Aggregates improve the soil

When clay particles are bound together in aggregates, the properties of the soil are almost always improved. An important effect is that soil tillage becomes easier. The movement of air is also better, improving the transport of oxygen to and carbon dioxide from the root system. The aggregate structure also improves soil permeability to water and its water-holding capacity. Good soil structure makes an arable clay soil a more productive growing site, since all essential functions for the plant and its root system are provided. This gives faster growth and higher yield.

Dictionary:

Texture = soil texture refers to the proportions of mineral particles with different average diameter, i.e. the relative proportions of sand, silt and clay in particular according to table “Particle size distribution” in chapter The building blocks of soil.

Single-grained structure = in a soil with single-grained structure, the primary particles are held together very weakly or not at all in some form of aggregate.

Aggregated structure = when the primary particles come from the clay particle size group, they are bound to each other and form aggregates that can then be stabilised by organic material, lime and various chemical precipitates.

Colloid = colloids are the finest particles in the soil, with an average diameter of less than 0.0002mm. The colloids include some organic material and fine clay.

Oxygen = element that occurs in air as oxygen gas O₂ at a concentration of 21 % – vital for cell respiration in plants and their roots.

Carbon dioxide = gaseous waste product (CO₂) of cell respiration in the roots that is also the building brick together with water for sugars created by the plant through photosynthesis.