12.2.2: Losing the Soil

Soil erosion is a geological process and a little bit of soil loss occurs naturally. In farming, it is possible to have no loss of soil, and in fact, to build soil. However, that's only possible when the plow is not used and perennial crops, such as well-managed pastures cover the fields. That is one reason to support 100% grass-fed cattle operations. Their soil loss is nil. Our goal should be to minimize erosion caused by farming operations while still feeding the world. There are ways to control the accelerated loss caused by the plow, tillage, and other cropping practices. Here we examine 'tolerable' soil loss.

Figure \(\PageIndex{1}\): A single raindrop impacts bare soil. Source: U.S. Dept. of Agriculture. Copyright info: Public Domain

The USDA Natural Resources Conservation Service (NRCS) is the federal agency charged with keeping soil on the land and out of the water. (It is the old soil conservation service). They do help farmers implement practices that help farmers do a better job of controlling erosion. The NRCS estimated maximum amount of soil that can be lost to erosion each year has been called the soil loss tolerance, or T value. This concept is used for qualifying NRCS-supported cost-share programs on farms. Practices used should bring estimated soil loss below the “tolerance” values estimated for the farm’s soils. For deep soil with a rooting depth of greater than 5 feet, the T value is 5 tons per acre per year (11 metric tons per hectare). Still, 5 tons is equal to about .03 inch of soil depth (about .08 centimeters), and if soil loss continued at that rate, at the end of 33 years about 1 inch would be lost. This “tolerable” soil loss rate is in essence a compromise and does not fully prevent soil degradation. On deep soils with good management of organic matter it takes many years to see a noticeable impact of soil loss. This is part of the concern: following these guidelines potentially diminishes long-term productivity.

Figure \(\PageIndex{2}\): A waterway scoured into a gully on a Midwestern cornfield after erosive spring rains. Photo by Andrew Phillips.

The soil loss “tolerance” amount is reduced for soils with less rooting depth. When rooting depth is less than 10 inches, the tolerable rate of soil loss is the same as losing 0.006 inches per year and is equivalent to 1 inch of loss in 167 years. Of course, on agricultural fields, the soil loss is not evenly distributed over the field, and greater losses occur in areas where runoff water collects and continues to flow (Figure \(\PageIndex{2}\)). When soil loss is greater than the tolerance value, productivity suffers in the long run. Yearly losses of 10–15 tons or more per acre occur in many fields. In extreme cases, as with croplands on steep slopes in tropical climates, losses of five or 10 times that much may occur.

Reducing erosion to the greatest extent possible can be achieved by combining practices that have many other positive effects aside from lessening soil loss. These practices include minimizing tillage, using cover crops, and following better rotations. Farmers creatively using such practices customized for their conditions can maintain soil productivity over the medium to long term even if a small amount of erosion occurs, as long as new topsoil can be created as rapidly as the soil is lost, estimated at about 0.5 tons per acre (about 1 ton per hectare).

Cost Of Erosion Per Bushel Of Crop

One way to look at the amount of erosion is to compare it to the amount of crops raised. For example, it is estimated that the average yearly soil loss from Iowa farms is about 5.5 tons per acre. Average Iowa yields are around 180 bushels of corn and 60 bushels of soybeans per acre. Using those values and assuming a 5.5-ton annual soil loss, there is approximately 1 pound of soil lost per pound of corn produced and 3.3 pounds of soil lost per pound of soybeans produced. We previously discussed the exporting of nutrients from farms that are integral parts of the crops sold. But this is another pathway for nutrients to leave the farm in relatively large quantities.