Shaping the Soil: The Human Impact on Organic Matter and Crop Rotation

 Human Influences on Soil Health

The loss of topsoil, which is rich in organic matter, due to erosion has significantly diminished the total amount of organic matter stored in many soils after they were developed for agriculture. Crop production inevitably suffers when a portion of the soil’s most fertile layer 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 more severe 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.

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