Soil Organic Matter
Soil Organic Matter Management
Any effort to improve soil quality and function needs to start with restoring soil organic matter, which is the primary influence on soil’s physical, biological, and chemical characteristics as discussed. Building organic matter is a slow process. First, quantity of labile organic matter generally responds to changes in management practices more quickly than the quantity of stable soil organic matter, so changes in labile organic matter levels can serve as a leading indicator of long-term trends in total organic matter levels. Gradually, the species and diversity of organisms in the soil will change, and amounts of stabilized organic matter will rise. It may take a decade or more for total organic matter levels to significantly increase after a management change. Fortunately, the beneficial effects of the changes appear long before organic matter levels rise. Changes in soil organic matter usually occur slowly and require at least three to five years to become measurable. The time required to achieve a measurable change in soil organic matter depends on many site-specific factors. Organic matter can be increased much more readily in soils that have become depleted of organic matter than in soils that already have a good amount of organic matter given their texture and drainage condition.
Soil Organic Matter Requirements
Soil organic matter requirements are likely to differ according to function and soil type. For example, for cation exchange capacity (CEC) soil organic matter is of greater importance in sandy soils compared with clayey soils. Soil organic matter is required in larger amounts in sandy soils because most clayey soils can provide a substantial proportion of CEC through charge derived from clay minerals.
Predicting Changes in Soil Organic Matter
Predicting changes in soil organic matter over time is an inexact science because the interactions among soil management practices, soil type, and variations in climate, and microclimate. A single soil management practice can alter the rate of both organic matter accumulation and loss. For example, a change from dryland to irrigated farming alters both organic matter inputs (irrigated crops produce more biomass) and the rate of organic matter decomposition (organic matter decomposition speeds up when soil is moist).
Strategies for Building Soil Organic Matter
Almost all soil and crop management practices have implications for soil organic matter. Practices influencing soil organic matter include tillage and planting techniques, methods of handling crop residue, application of organic amendments, crop rotations, and the use of cover crops. Soil and crop management practices usually influence all these soil organic matter fractions. To effectively increase soil organic matter (i.e., carbon), the rate of input must exceed the rate of loss from decomposition and leaching processes.
Conservation Tillage
Conventional tillage is a system that traditionally uses moldboard plows or chisel plows with sweeps, followed by disking, harrowing, or other secondary tillage operations to incorporate residue, prepare a seedbed and control weeds (Figure 9.4). Conventional increases the soil surface area, allowing more air into the soil and speeding the decomposition rate. Tilling crop residue into the soil also makes residues more accessible to soil organisms and incorporates oxygen into the soil, increasing the decomposition rate of the residues and decreasing active fraction of soil organic matter at the soil surface and plow layers.
Organic Amendment Considerations
Organic materials can also be added by amending the soil with composts or animal manures. The addition of organic amendments is particularly important in vegetable production where minimal crop residue is returned to the soil, more intensive tillage is generally used, and land is more often a limiting factor making the use of cover crops more challenging. Various organic amendments can affect soil physical, chemical and biological properties quite differently, so decisions should be based on identified constraints and soil health management goals.
Reduce Fallow Periods
Seasonal fallows–periods with no living plant cover–are frequently implemented in dryland cropping systems to allow recharge of soil moisture for the subsequent cash crop. Soils devoid of plants have no input of organic matter and soil microbes continue to metabolize remaining soil organic matter into carbon dioxide. Over time, repeated fallow periods deplete soil organic matter that affects the long-term functional capacity of soils.
Cover Crops
Cover crops are able to increase soil organic matter by adding biomass to the soil (especially below the soil surface). A cover crop is termed a green manure crop if the residue is incorporated in the soil. Farmers usually plant cover crops in the fall and till them as green manure in the spring, before planting. The earlier cover crops are planted, the more benefits they will provide. Many types of plants can be grown as cover crops in the crop rotation scheme.
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