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The Importance of Organic Matter in Soil Fertility and Crop Health
This article was first printed in the Nov/Dec 2009 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
One of the greatest challenges producers face when beginning organic management is providing adequate fertility to meet crop needs. Synthetic fertilizers provide nutrients in an inorganic form, and are therefore immediately available for uptake by the crop. In the absence of these fertilizers, organic nutrient sources are needed to supply fertility. These sources require processing by the soil microbial community before plants can utilize them. Soils that have been under conventional management often do not support the levels of organic matter to supply plant nutrients, or an active microbial community to efficiently process those nutrients and make them available to the crop. However, careful planning of fertility programs can alleviate nutrient deficiencies that may occur in the transition years, as well as help to build healthy, disease and pest resistant soils and crops.
Soil organic matter is the most fundamental source of fertility in organic agriculture and it is important for producers to understand the basics of organic matter cycling in the soil. Soil organic matter is that portion of the soil that consists of biological residues, from plant to animal to microorganism, in various stages of decay. These residues are decomposed by soil fauna, including relatively large organisms such as earthworms (macrofauna), nematodes and springtails (mesofauna) and microorganisms (fungi and bacteria). Depending on the carbon to nitrogen (C:N) ratio of the residues, the fate of the decomposition products are different. High carbon residues such as corn stalks decompose slowly, because a lack of nitrogen limits the ability of microorganisms to break the material down. The majority of the nitrogen that is extracted from these residues is immediately incorporated into the bodies of the soil fauna, therefore making it unavailable to the plant, at least for the time being. High nitrogen residues such as legumes, on the other hand, will decompose quickly and, due to an excess of nitrogen in relation to the needs of the microorganisms, release nitrogen that is readily available for plant uptake.
The various components of plant and animal residues also have various fates. Certain parts may break down easily, liberating nutrients, while other portions will continue to be worked on and altered by microorganisms until they can no longer be broken down. At this point these materials are called humus. Humus is extremely important in increasing and maintaining soil fertility. It possesses an overall negative charge, which translates into a very high cation exchange capacity. This means it is able to attract and effectively store positively charged ions, or cations. As the majority of plant macro and micronutrients in the soil (with the notable exception of phosphorus) are cations, humus can be thought of as a bank which holds nutrients and releases them in response to plant or microorganism secretions. Additionally, nutrients in the soil are subject to a complex array of chemical reactions and these also affect their absorption and release.
From the above we can see that soil organic matter is the heart of balanced soil fertility. Organic matter supplies not only readily available nutrient sources but also the building blocks of humus. Including a broad selection of crops in a rotation ensures diverse sources of organic matter, and is an important strategy for increasing the overall organic matter content of the soil. The inclusion of combinations of materials with low and high C:N ratios is very important. Low C:N ratio materials, because they contain relatively large amounts of nitrogen, decompose quickly, contributing very little to the building of humus. High C:N ratio residues, on the other hand, break down more slowly in the soil due to the presence of more stubborn compounds. These residues increase soil organic matter and humus contents but contribute relatively fewer readily available nutrients. Therefore a diversity of crop residues ensures sufficient organic C and N for humus formation and ultimately produces a pool of potentially available nutrients that can become mobilized according to crop demand.
Soil microfauna and microorganisms mediate the release of these nutrients, and diverse residue sources sustain a microbial community that is more efficient and has more functional diversity. For instance, bacterial biomass is associated with readily available organic matter fractions, while the abundance of fungi increases in relation to the materials with higher C:N ratios. In general, increasing soil C is linked to greater soil microbial biomass, which is an important sink and source of nutrients. Although the incorporation of nutrients into living microbial biomass can, at least initially, reduce availability for plant uptake, over time the cycling of nutrients through microbial biomass should reach equilibrium, at which point nutrients are readily available for crop needs. This process is hastened by the presence of predators, such as bacteria-feeding nematodes, which have been shown to double the rate of N release.
Furthermore, it is estimated that 20 to 70% of the soil cation exchange capacity is due to humus, which highlights the importance of organic matter for nutrient storage. High organic matter contents also have a positive effect on soil physical properties. For example, soils with high organic matter contents contain a greater abundance of water-stable aggregates and have a greater exchange capacity, which translates into better structure and water-holding and nutrient absorption capacities. Larger aggregates also slow organic matter degradation, producing a slowly mineralizing pool of nutrients.
Organic matter can also reduce crop attractiveness to insect pests. In fact, plants growing in soils receiving diverse organic matter inputs have been shown to be less attractive to some insect pests, as a result of a more nutritionally-balanced growth medium. The effect of fertilization based predominantly on one nutrient out of balance with other essential nutrients often leads to an environment attractive to insect pests. In fact, inorganic fertilizers, most commonly nitrogen, are known to increase plant predation. This is due to the response of the plant to nutrient imbalances. A plant grown in mineral-balanced soil will initially produce simple metabolic compounds, such as amino acids and sugars, which are subsequently assimilated into secondary metabolic compounds that selectively promote (1) vegetative/reproductive growth and (2) enhanced insect and disease resistance. On some level a trade-off must be made between these two pathways; however, in mineral-balanced soil these pathways are interrelated and positively correlated. In environments with an excess of readily available N, on the other hand, the plant will accumulate a large amount of simple compounds, effectively unable to metabolize these compounds further due to the nutrient imbalance. So not only do the absence of secondary metabolites reduce pest resistance, but the simple compounds are metabolically more accessible to insect herbivores. These simple compounds act as feeding and egg-laying stimuli for many herbivorous insects, and therefore it is no surprise that the development and fitness of these insects is linked to their abundance.
In many cases, healthy soils can also promote the suppression of common soil-borne crop diseases. Two types of suppression, general and specific, work to inhibit the activity and fitness of disease-causing agents. Many plant pathogens are poor competitors in the soil and therefore general suppression of these pathogens results from competition for resources by other non-pathogenic microorganisms. Interspecies relations such as amensalism (a relationship between two species where species A negatively impacts the fitness of species B without gaining any benefit) and non-selective predation also help to define general suppression. No single species is responsible for general suppression, rather the community as a whole acts as an overall regulator of the individual populations. Therefore, this type of suppression is a result of a diverse microbial community, and can effectively lead to biostatis, or conditions which disfavor the inordinate growth of any specific species. Under these conditions it is likely that populations of pathogenic microorganisms will be held at levels below those necessary for a disease outbreak to occur. Ultimately, a soil system that is nutrient deficient, or lacks the proper mineral balance, will often lack an active microbial community, creating inefficiencies and imbalances in the community which pathogens can exploit.
While general suppression stems from the dynamics of the entire microbial community, specific suppression describes the direct antagonism of a pathogen by a non-pathogen at some point in the life cycle of either species, usually through predation. There is much study still to be done on mechanisms of specific suppression, which can at times be difficult to separate from general suppression.
Soil fertility can also be related to weed abundance. Increasing organic matter content has been found to be related to decreased weed abundance due to a higher abundance of bacteria toxic to germinating seeds. In fact, direct microbial predation has been determined to be a significant fate of weed seeds in the soil. Weed seed predation by microarthropods and invertebrates such as crickets and beetles is also extremely significant and is enhanced by increasing ground cover. The distribution of weeds in a field also has some links with varying soil properties. Weeds are evolutionarily endowed with the ability to adapt to and survive in a vast array of soil conditions, which is much of the reason that simple relationships between weeds and soil properties can be difficult to observe consistently. However, it is generally accepted that any plant evolves in such a way that optimal conditions produce optimal fitness; therefore, it seems extremely likely that differing soil conditions may favor certain species of weeds, at the expense of either the growing crop or other weeds. In fact, the competitive advantage of a weed is probably less likely to be related to a singular soil property but instead to the ratio of one nutrient concentration to any number of other nutrient concentrations, or the interaction of various soil physical properties.
The benefits of healthy soils to crops are many, and management is the key to ensure that a soil is functioning correctly. Practices that can help to build healthy soils include crop rotation, organic matter additions, or using high-residue tillage implements. The inclusion of green manures and cover crops in a rotation is an excellent way to sponsor fertility, suppress weeds and provide a break in pest cycles. Incorporating several different species of crops in a rotation, along with manures and/or compost, ensures a diversity of organic matter sources. This diversity leads to a more minerally-balanced soil and a pool of nutrients which become available slowly over time, reducing leaching, waste and toxicity that can result from immediately-available inorganic fertilizer additions. Ultimately, managing for good soil fertility is extremely important because the soil environment and the surrounding air environment are in reality virtually inseparable, and the establishment of a functional and stable system in one environment can have far-reaching impacts in the other.
Jeff Gunderson, the new outreach coordinator at MOSES, has a Master's Degree in soil science and a background in agronomy. He may be contacted at firstname.lastname@example.orgReturn to TOP