Chapter 4

Soil Management on Organic Farms

Soil Organic Matter

Soil organic matter plays an important role in integrating many aspects of soil health. Soil organic matter is that portion of the soil that consists of carbon-based 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, 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

For simplicity, organic matter can be divided into two major categories: stabilized organic matter, which is highly decomposed and stable, and the active fraction that is partially decomposed, which includes components of living plants, animals, and soil organisms.

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 that 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 referred to as the “stable fraction” and what we commonly refer to as “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.

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

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). Liquid manure may not contribute significantly to organic matter if it does not contain any solids. Highly decomposed packed manure or compost may contribute significantly to the stable pool of organic matter since it is already partly decomposed and stabilized.

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) carbon to nitrogen ratio.

Soil Moisture and Temperature. Decomposition of soil organic matter and the subsequent release of inorganic nitrogen from the organic nitrogen pool occurs through the activity of soil microflora, principally bacteria and fungi. Soil temperature and moisture content have a strong effect on nitrogen mineralization reactions.

Soil Texture and Mineralogy. The amount and type of clay in a soil affects nitrogen mineralization reactions. Mineralization tends to be greater in coarse-textured soils low in clay and less as the soil clay content increases.

Tillage Practices. Tillage enhances the mineralization of soil organic carbon and nitrogen by incorporating crop residues, disrupting soil aggregates, and increasing aeration.

Carbon to Nitrogen Ratio. The carbon to nitrogen ratio of organic matter means the amount of carbon relative to the amount of nitrogen present. There is always more carbon than nitrogen in organic matter. The carbon to nitrogen ratio is usually written as C:N and is a single number, because it expresses the ratio of carbon to nitrogen. For example, if the ratio is 20, this means that there are 20 grams of carbon for each gram of nitrogen for that organic matter.

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