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CONSEQUENCES FOR ORGANIC MATTER IN SOILS

http://people.oregonstate.edu/~muirp/orgmater.htm

Another problem with extensive and abundant use of inorganic fertilizers is that their use does not improve soil fertility and structure over the long term. Further, as they are used more and more in place of alternative practices that do build the soil, we become increasingly dependent on their use — an addiction.

WHERE DO NUTRIENTS COME FROM IN NATURAL ECOSYSTEMS?

Largely from four sources, whose relative importance varies with the ecosystem:

Inputs from the atmosphere (both with precipitation and in dry form)

Weathering of the parent material underlying the soils

Activity of nitrogen-fixing organisms

Decomposition of dead organic matter

Let’s focus on the last of these. How does decomposition affect soil structure and fertility? Following is quite a simplified version of these complex processes.

(1) An organism dies (or parts of it do) and is on or in the soil.

(2) Invertebrates, fungi, and bacteria living in and on the soil consume it.

(3) In so doing, they assimilate many of its nutrients into themselves. This process is called “nutrient immobilization.” As long as these nutrients are in the bodies of the decomposers, they are not available for plant uptake, hence the term “immobilization;” they are temporarily unavailable. When the nutrients are immobilized, they are in organic form. (Decomposers can also immobilize nutrients that are applied inorganically and tie them up temporarily in organic form. This is particularly common with nitrogen, and particularly when the carbon:nitrogen ratio of the litter is very high, such that nitrogen is in great demand.) You might have experience with this phenomenon if you are a gardener. Have you ever applied an abundance of carbon-rich mulch, such as sawdust, to your garden, hoping to enhance its organic matter content, but then noticed that the plants don’t seem to be producing as well as usual — and may look yellowed? If so, it is likely that the decomposers who were stimulated by the addition of all that good carbon-containing material have immobilized most of the available nitrogen (and possibly other nutrients) in the process of decomposing the material, so that less is available for the plants.

Interesting sidelight: you probably know that the invasive Eurasian grass cheat grass (Bromus tectorum) has taken over vast areas of rangeland, particularly in the western US? Well, researchers have learned that it has higher nitrogen requirements than do many of the native rangeland plants, and experiments are underway to take advantage of this. I kid you not — researchers are spreading table sugar on infested rangelands (often in association with other treatments as well), attempting to decrease nitrogen availability through the mechanism I described just above, hoping that this will diminish the competitiveness of cheat grass in these systems. Dr. David Pyke of the USGS (and affiliated with OSU’s Department of Botany and Plant Pathology) is involved in some of these research efforts.

(4) The decomposers all tend to be short-lived. They die and are consumed by others, often through a complex succession of organisms. However, gradually, the nutrients that they contained are converted to inorganic forms either through biological action or by leaching by water. This process is calledmineralization; the conversion of organically bound forms of nutrients to inorganic forms as result of inorganic or biological chemical reactions. At this point, the nutrients are available for plant uptake.

Thus, in soils, death, immobilization and mineralization, coupled with plant uptake, are constantly occurring. Nutrients gradually become available from the decay of organic inputs, which act essentially like timed-release fertilizers. This is the nutrient cycling with which you are undoubtedly already familiar.

One result of this process is that, in natural ecosystems, nutrient cycling (particularly of nitrogen, which is most commonly a limiting nutrient)– is very “tight.” Not much is lost from the soil because:

(1) it is released slowly,

(2) it is taken up rapidly, and

(3) there is usually little loss of nutrients via erosion in natural ecosystems. Some nutrients are lost, of course, as with erosion of soil, volatilization to the atmosphere, and leaching with water, but losses are usually minor.

Natural ecosystems also generally have a high storage capacity for nutrients. This storage capacity comes largely in and on organic material which is slowly decayed.

ROLES OF ORGANIC MATTER (THE HUMUS CONNECTION)

The least decomposable material is left behind as humus, a structureless, dark, chemically complex organic material. Humus is also gradually decomposed, releasing nutrients, but also is continuously added to in intact systems

Humus is vitally important for soils:

(1) It provides nutrients as it is decomposed and an energy source for soil organisms.

(2) It furnishes sites for retention of other nutrients. Humus contains large numbers of charged sites – for example negatively charged sites to which positively charged nutrient ions (such as ammonium [NH4 + ]) adhere.

(3) It increases the water-holding capacity of soil (is “spongy”).

(4) It improves soil aeration, as it is fibrous and porous. This improves the soil environment for many microbes and plant roots, which require aeration.

(5) It increases infiltration of water into the soil (versus run off of water), again because of its porosity.

The latter three all related to aspects of the soil “structure” which are enhanced by organic matter in soils.

So, what does all this have to do with inputs of inorganic fertilizers? First:

WHERE WERE HISTORICAL SOURCES OF ORGANIC MATERIAL IN FARMED SOILS BEFORE WE BEGAN RELYING ON INORGANIC INPUTS?

(1) Many of our richest farm soils are rich because for thousands of years they supported productive prairies, which accumulated soil organic matter over the ages. Hence, current agriculture in many cases relies on this historical, pre-agricultural accumulation of organic material.

(2) Historically, agricultural systems featured mixed crops and livestock. The animals were used to work the soil (as for plowing) and also grazed on crop residues post-harvest. Their manure furnished organic matter to the soils.

(3) Relatively abundant crop residue was left on the site post-harvest, which also contributed to soil organic material.

(4) Farmers made extensive use of fallow years (years when the soil was left unplanted, to recharge its water and nutrients) and of crop rotation (rotating, for example, a legume (which fixes atmospheric nitrogen) with a grain, and plowing in the legume (or its residue) at grain planting time or before, and thus adding nitrogen and organic material to the soils). The effectiveness of rotation of course depends on the species being used and how much of them is harvested before tillage or re-planting. For example, soy doesn’t add much organic matter to the soil because it doesn’t produce much post-harvest residue – (see Bullock’s article on rotation in the supplementary reading list for more information on this).

MODERN AGRICULTURE MINIMIZES THESE INPUTS:

First, farmers find it easier to regulate precisely the amounts of various nutrients added to the soil by using inorganic fertilizers. They are also easier and less time- and labor-intensive to use than organic sources, and the yield gains tend to be more immediate for inorganics than for organics, which take time to decompose and release nutrients. Other changes in agriculture that contribute to the decreased emphasis on organic sources of fertility include:

(1) Fewer farms mix livestock and crops (there is more specialization in agriculture). Much of the manure from feedlots does get returned to the soil, but only after it has lost, typically, much of its nutrient value. Also, most manure is returned to the fields that are close to the feedlots (which then receive more nutrients than they can retain…) because of the perception that it costs too much to haul it farther — we’ll examine that assumption later.

(2) Harvests are often more intensive than in the past. For example, crop residues are removed and used for animal feed off-site, and more and more of them are being used for bio-energy production, straw bale construction, and so forth. Furthermore, the Green Revolution varieties produce less residue than did traditional varieties, in general (recall the high “harvest index” associated with Green Revolution varieties?)

(3) There is decreased use of rotation and fallow. Land tends to be under continuous cultivation of single crops — for example, in 1991, 40% of US corn acreage was being grown continuously in corn.

WHY HAS USE OF CROP ROTATION DIMINISHED?

(1) For many years, US government farm policies essentially blocked farmers from rotating. To receive full crop subsidies and other financial supports, growers had to commit acreages to certain crops, which made rotation a financial liability. (See the National Academy of Science book on the supplementary reading list for information on this.) The 1990 and 1996 Farm Bills eased these restrictions a bit, but there are still restrictions. (See 2002 Farm Bill for information on the conservation provisions for the 2002 version of the Bill; information on the 2008 Farm Bill is provided part way down the section of notes that deals with Conservation Tillage.) Subsidies are hugely important in US agriculture — during the 1990’s, ~ 1/4 of net farm income for US agriculture came from direct government payments. However, most rotation crops are not eligible for subsidies — the ’08 Farm Bill greatly limited the number of crops that are eligible for such payments; 90% of payments went to coren, wheat, soy, cottom and rice. (Side note, ~ 70% of these subsidies do not go to regular family farm operations but rather to the largest 10% of producers, which are often corporations such as Cargill. Many small farmers do, however, depend on these subsidies, so can’t afford to use rotation crops.)

(2) There is a perceived need in these times to have every acre be producing food crops every year, which makes rotation with many legumes less attractive than it might otherwise be

(3) Increased mechanization decreased the need to raise rotation crops (e.g., alfalfa) as feed for animals, since fewer farmers used animals to work the farm and there are fewer mixed livestock-crop operations.

(4) Mechanization and the economies of scale associated with specializing worked against rotation. A grower needs less equipment if he/she is only growing corn than if he/she is growing corn and a rotation crop.

(5) Economics have worked against rotating in another way as well, in that rotating requires that the land be used every other year or so for crops that may not bring as much money as the grain crops.

SO, WHAT HAPPENS OVER TIME AS INORGANIC FERTILIZERS ARE USED IN PREFERENCE TO ORGANIC SOURCES OF FERTILITY?

A series of interconnected changes occur as we rely more and more on inorganic inputs and less and less on organics.

(1) Humus decreases because:

There is less organic input

Soil microbes that were nutrient limited can decay what organic matter there is faster when inorganic nutrients are supplied. (This faster decay persists at least until structural problems in the soil [described below] outweigh the nutrient advantages. That is, early on, inorganic inputs increase decomposition rates, but, over time, that often reverses.)

Intensive tillage disrupts soil aggregates (clumps), so there is faster decomposition of organic material. This involves both faster chemical oxidation and enhanced activity by aeration-stimulated decomposers.

A dramatic example of this loss of organic material in agricultural soils is in the midwestern US, whose prairie soils have lost 1/3 – 1/2 of their organic material since they began being cultivated. This is a common pattern: conversion to cropland is almost universally associated with a rapid decrease in soil organic matter and soil nitrogen content.

(2) Lowered organic material (humus) results in:

Decreased input of slow-release fertilizer

Lowered water holding capacity and more runoff of water (which takes with it inorganic fertilizers and soil with its organic material [which is typically concentrated in the topsoil, which is most vulnerable to erosion by runoff]).

Decreased aeration, as the soil loses the structure given by organic material. This of course worsens problems of increased runoff. Poorly aerated soils are also less suitable for beneficial soil organisms, so natural inputs of nutrients via mineralization eventually slow. In addition, poorly aerated soils are not as good for plants, worsening the efficiency of their roots at taking up nutrients

(3) Heavy use of inorganic sources of fertility, in particular nitrogen fertilizers, decreases the efficiency of free-living nitrogen fixers in the soil. Thus, their fixation rates decrease and there is less natural input of nitrogen.

(4) High inputs of nitrogen fertilizers can also result in soil acidification.

Essentially, as growers add inorganic fertilizers without due attention to organics, they step onto a one-way street. The combination of factors described above means that they need to add ever-increasing amounts of inorganic fertilizers to sustain their yields. It is similar to any addiction, where increasing amounts of the desired substance are required to achieve satisfaction.

The amounts of inorganic fertilizers required increase because natural inputs of fertility and the nutrient retentiveness of the system diminish:

Humus is lost rapidly (see above)

Biological nitrogen fixation slows

Soils are less retentive of nutrients because: (a) enhanced runoff takes more nutrients off with it (b) lowered organic material means fewer exchange sites in the soil on which nutrients are normally retained, and (c) plant roots are less efficient in poorly aerated soils.

Thus, Green Revolution style agriculture (in company in the US with the requirements of government farm subsidy programs) has set farmers up for a system that requires increasing dependence on inputs of inorganic fertilizers, rather than on methods of insuring soil fertility that are potentially more sustainable and also less energy intensive. (Remember all that fossil fuel energy that is required to make nitrogen fertilizers….)