HUMAN INFLUENCES
HUMAN INFLUENCES 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. Organic matter also is lost from soils when organ-isms 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 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 amend-ments 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 unusu-ally high reserves of soil organic matter and nutrients at the time of conversion to cropland. Rapid decomposition of organic matter by organ-isms usually occurs when a soil is intensively tilled. Incorporating residues with a moldboard plow, breaking aggregates open, and fluffing up the soil allow microor-ganisms to work more rapidly. It’s something like open-ing 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 begin-ning 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 microorgan-isms. 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 chan-nels that also help water infiltrate into the soil. The ben-eficial 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 accumula-tion 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.
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 amend-ments 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 unusu-ally high reserves of soil organic matter and nutrients at the time of conversion to cropland. Rapid decomposition of organic matter by organ-isms usually occurs when a soil is intensively tilled. Incorporating residues with a moldboard plow, breaking aggregates open, and fluffing up the soil allow microor-ganisms to work more rapidly. It’s something like open-ing 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 begin-ning 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 microorgan-isms. 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 chan-nels that also help water infiltrate into the soil. The ben-eficial 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 accumula-tion 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.
