Soil Management on Organic Farms
Soil Organic Matter
Soil organic matter plays an important role in integrating many aspects of soil health (See Figure 4.1). Soil organic matter is that portion of the soil that consists of carbonbased compounds from the remains of plants, animals, and microorganisms. Soil organic matter influences many soil properties including infiltration rate, bulk density, aggregate stability, cation exchange capacity (CEC), and biological activity, all of which are related to a number of key soil functions. Soil organic matter serves as a slow-release reservoir for plant macronutrients (especially nitrogen) and also aids in plant micronutrient nutrition. It buffers and neutralizes soil pH, facilitates the infiltration of water and air into the soil, increases water retention by the soil, and is important in maintaining soil tilth. Over time, increases in soil organic matter can lead to a larger and more diverse population of soil organisms and may thus enhance the biological control of pests and plant diseases. Large quantities of fresh organic matter that are added to the soil, however, may stimulate plant pathogenic organisms and seed and seedling pests such as cabbage maggots and wireworms, which can cause serious losses.
Benefits of Soil Organic Matter
In addition to supplying nutrients, soil organic matter improves soil fertility by imparting favorable chemical and physical attributes to soil. Soil organic matter can bind nutrients through the process of cation exchange. Ammonium, calcium, magnesium, and potassium are nutrient cations that are held on cation exchange sites on organic matter. The cation exchange capacity of soil organic matter can contribute from 20 to 70 percent of the total cation exchange capacity of soil.
Classification of Organic Matter
Several types, or “pools,” of soil organic matter exist, each important for different soil functions. Think of these pools of soil organic matter on a continuum from fresh, active organic matter that is gradually transformed to well-decomposed humus.
Active Fraction
The active fraction of organic matter is composed of a range of material, including recent plant litter and highly decomposed unrecognizable plant and other organic residues that break down in a very short time, from a few weeks to a few years. This kind of organic matter is associated with a lot of biological activity. The active organic matter accounts for only a small fraction of the total organic matter in the soils but is much more sensitive to soil management practices and is closely related to nutrient cycling and soil tilth-promoting functions of soil organic matter.
Stable Fraction
Many soil organisms assist in the process of decomposing plant and animal tissues. During the process of decomposition, chemical transformations take place, creating new organic compounds in the soil. These organic compounds are relatively resistant to decomposition because of either their chemical structure, their adsorption to clay particles, or their protection within micro-aggregates. This chemical complex is referred to as the stable fraction, also known as humus, and is not very biologically active. Humus usually represents most of the total soil organic matter and is relatively stable over time. It can take hundreds to thousands of years to fully decompose. Its presence can be recognized by the dark color it confers to soil, and the crumbly or slightly gelatinous texture and characteristic “earthy” smell of the soil. Humus is responsible for giving the soil that rich, dark, spongy feel.
Factors Influencing the Quantity of Soil Organic Matter
The quantity of organic matter in a given soil is the result of a balance between organic matter inputs, such as crop residues, manure, and compost, and the rate of organic matter decomposition. Organic matter inputs can be influenced by crop management, such as the use of cover crops, crop rotations, and residue management, as well as soil management, such as using organic forms of nutrients like compost and manure.
Increases in Organic Soil Matter
Management practices can alter the diversity and complexity of interactions in the soil through their effects on organic matter. To maintain the same level of active soil organic matter requires a constant supply of fresh organic materials. For example, organic matter can be increased by growing crops that produce large amounts of residue and fine roots (species such as corn, small grains, grasses), leaving crop residue in the field, growing cover crops during otherwise bare fallow periods, and adding compost and manure (especially bedded manure, which has a greater concentration of organic matter than liquid manure).
Decreases in Soil Organic Matter
Several factors contribute to the decomposition and loss of soil organic matter. Organic matter loss is accelerated by crop residue (such as hay, straw, or silage harvest) removal or burning and tillage, which redistributes organic matter in the soil profile and increases organic matter decomposition rates.
Mineralization
Organic matter mineralization is a process where microbes metabolize organic carbon and convert organic nitrogen compounds into simple inorganic ions (nitrate and ammonium), which are available for plant uptake. Sources of mineralizable organic nitrogen include native soil organic matter, crop residues, and organic amendments like manure and compost. Initially, larger organic matter molecules are broken down into smaller ones, with soil microorganisms attacking these remaining materials by producing specific enzymes. The transformation of organic nitrogen to the ammonia (NH3) and ammonium (NH4+) forms is referred to as ammonification.
Factors Affecting the Rate of Mineralization
Nitrogen mineralization is always coupled with immobilization in the soil environment, and a complex set of factors determines whether nitrogen is released for plant uptake or remains immobilized by soil microbes. Mineralization rates are affected by soil temperature and moisture conditions as well as by the accessibility and nature of the organic. As ammonium or nitrate are released, both are assimilated rapidly by soil microbes to oxidize new carbon substrates (i.e., nitrogen is immobilized). However, this nitrogen will subsequently be available for mineralization as the soil microbe population turns over. Estimating and predicting the amount and timing of nitrogen mineralization is complicated because of the numerous factors affecting the process. The most important of those factors are: (1) soil moisture temperature, (2) soil texture and mineralogy, (3) tillage practices, and (4) carbonto- nitrogen ratio (C:N).
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