Loss of topsoil that is rich in organic matter by erosion has dramatically reduced the total amount of organic matter stored in many soils after they were developed for agriculture. Crop production obviously suffers when part of the most fertile layer of the soil is removed. Erosion is a natural process and occurs on almost all soils. Some soils naturally erode more easily than others, and the problem is greater in some regions than others. However, agricultural practices accelerate erosion. It is estimated that erosion in the United States is responsible for annual losses of about a billion dollars in available nutrients and many times more in total soil nutrients.
Unless erosion is severe, a farmer may not even realize a problem exists. But that doesn’t mean that crop yields are unaffected. In fact, yields may decrease by 5% to 10% when only moderate erosion occurs. Yields may suffer a decrease of 10–20% or more with severe erosion. The results of a study of three midwestern soils (referred to as Corwin, Miami, and Morley), shown in table 3.1, indicate that erosion greatly influences both organic matter levels and water-holding ability. Greater amounts of erosion decreased the organic matter content of these loamy and clayey soils. In addition, eroded soils stored less available water than minimally eroded soils.
Table 3.1: Effects of Erosion on Soil Organic Matter and Water
|Soil||Erosion||Organic Matter (%)||Available Water Capacity (%)|
Source: Schertz et al. (1985).
Organic matter also is lost from soils when organisms decompose more organic materials during the year than are added. This occurs as a result of practices that accelerate decomposition, such as intensive tillage and crop production systems that return low amounts of residues. Much of the rapid loss of organic matter following the conversion of grasslands to agriculture has been attributed to large reductions in residue inputs, accelerated mineralization of organic matter because of plowing, and erosion.
Tillage practices influence both the amount of topsoil erosion and the rate of decomposition of organic matter. Conventional plowing and disking of a soil to prepare a smooth seedbed break down natural soil aggregates and destroy large, water-conducting channels. The soil is left in a physical condition that is highly susceptible to wind and water erosion.
The more a soil is disturbed by tillage practices, the greater the potential breakdown of organic matter by soil organisms. During the early years of agriculture in the United States, when colonists cleared the forests and planted crops in the East and farmers later moved to the Midwest to plow the grasslands, soil organic matter decreased rapidly. In fact, the soils were literally mined of this valuable resource. In the Northeast and Southeast, it was quickly recognized that fertilizers and soil amendments were needed to maintain soil productivity. In the Midwest, the deep, rich soils of the tall-grass prairies were able to maintain their productivity for a long time despite accelerated loss of soil organic matter and significant amounts of erosion. The reason for this was their unusually high reserves of soil organic matter and nutrients at the time of conversion to cropland.
Rapid decomposition of organic matter by organisms usually occurs when a soil is intensively tilled. Incorporating residues with a moldboard plow, breaking aggregates open, and fluffing up the soil allow microorganisms to work more rapidly. It’s something like opening up the air intake on a wood stove, which lets in more oxygen and causes the fire to burn hotter. In Vermont, we found a 20% decrease in organic matter after five years of growing corn on a clay soil that had previously been in sod for decades. In the Midwest, many soils lost 50% of their organic matter within forty years of beginning cropping. Rapid loss of soil organic matter occurs in the early years because of the high initial amount of active (“dead”) organic matter available to microorganisms. After much of the active portion is lost, the rate of loss slows and what remains is mainly the already well-decomposed “passive” or “very dead” materials. With the current interest in reduced (conservation) tillage, growing row crops in the future should not have such a detrimental effect on soil organic matter. Conservation tillage practices leave more residues on the surface and cause less soil disturbance than conventional moldboard plow–and–disk tillage. In fact, soil organic matter levels usually increase when no-till planters place seeds in a narrow band of disturbed soil, leaving the soil between planting rows undisturbed. Residues accumulate on the surface because the soil is not inverted by plowing. Earthworm populations increase, taking some of the organic matter deeper into the soil and creating channels that also help water infiltrate into the soil. The beneficial effects of minimizing tillage on soil organic matter levels are often observed quickly at the soil surface; but deeper changes are much slower to develop, and depletion at depth is sometimes observed. In the upper Midwest there is conflicting evidence as to whether a long-term no-till approach results in greater accumulation of soil organic matter (SOM) than a conventional tillage system when the full profile is considered. In contrast, significant increases in profile SOM have been routinely observed under no-till in warmer locations.
Crop Rotations and Cover Crops
Levels of soil organic matter may fluctuate during the different stages of a crop rotation. SOM may decrease, then increase, then decrease, and so forth. While annual row crops under conventional moldboard-plow cultivation usually result in decreased soil organic matter, perennial legumes, grasses, and legume-grass forage crops tend to increase soil organic matter. The high amount of root production by hay and pasture crops, plus the lack of soil disturbance, causes organic matter to accumulate in the soil. This effect is seen in the comparison of organic matter increases when growing alfalfa compared to corn silage (figure 3.3). In addition, different types of crops result in different quantities of residues being returned to the soil. When corn grain is harvested, more residues are left in the field than after soybeans, wheat, potatoes, or lettuce harvests. Harvesting the same crop in different ways leaves different amounts of residues. When corn grain is harvested, more residues remain in the field than when the entire plant is harvested for silage or stover is used for purposes like bioenergy (figure 3.4).
Soil erosion is greatly reduced and topsoil rich in organic matter is conserved when rotation crops, such as grass or legume hay, are grown year-round. The permanent soil cover and extensive root systems of sod crops account for much of the reduction in erosion. Having sod crops as part of a rotation reduces loss of topsoil, decreases decomposition of residues, and builds up organic matter by the extensive residue addition of plant roots.
Use of Synthetic Nitrogen Fertilizer
Fertilizing very nutrient-deficient soils usually results in greater crop yields. A fringe benefit of this is a greater amount of crop residue—roots, stems, and leaves— resulting from larger and healthier plants. However, nitrogen fertilizer has commonly been applied at much higher rates than needed by plants, frequently by as much as 50%. Evidence is accumulating that having extra mineral nitrogen in soils actually helps organisms better decompose crop residues—resulting in decreased levels of soil organic matter. (See chapter 19 for a detailed discussion of nitrogen management.)
Use of Organic Amendments
An old practice that helps maintain or increase soil organic matter is to apply manures or other organic residues generated off the field. A study in Vermont during the 1960s and 1970s found that between 20 and 30 tons (wet weight, including straw or sawdust bedding) of dairy manure per acre were needed to maintain soil organic matter levels when silage corn was grown each year. This is equivalent to one or one and a half times the amount produced by a large Holstein cow over the whole year. Varying types of manure—like bedded, liquid stored, digested, etc.—can produce very different effects on soil organic matter and nutrient availability. Manures differ in their initial composition and also are affected by how they are stored and handled in the field—for example, surface applied or incorporated.