SA Soil Erosion

 

The following article was adapted from an article entitled “More than 50 billion tons of topsoil have eroded in the Midwest” (Elizabeth Gamillo). The estimate of annual soil loss is double the rate of erosion USDA considers sustainable.

Soil scientist estimates that 57.6 billion tons of topsoil has been lost in the USA in the last 160 years. During the Dust Bowl era (1930’s), over 20 tons of topsoil per acre were lost annually in the Midwest due to wind erosion. Due to soil conservation efforts, erosion rates declined to around 7.4 tons nationwide and new estimates are closer to 5 tons per year.

However, these are only estimates and sometimes the way these numbers are calculated differs. In many cases, they are looking at only sheet, rill, and wind erosion; ignoring the gully erosion which is the most severe. Sheet erosion is the thin layer of topsoil that erodes across the whole field and is barely noticeable. Rill erosion occurs when water runs off and forms small channels as it moves downslope. Rill erosion can be up to 1 foot deep. Gully erosion is the worst type of soil erosion, the deepest and most severe (1-30 feet deep).

Researchers from Iowa Natural Heritage Foundation helped identify sites in the Midwest for this study. Researchers compared cropland next to the small isolated natural prairie sites. On average, farmed fields were 1.2 feet below the prairie, per Science News. Researchers found the average soil erosion rate was 1.9 millimeters over 160 years. A 1.9 millimeters per year or .0748 inch per year erosion rate is about equal to 11 tons/acre/year. The USDA T-factor is 4-5 ton/year which is an erosion rate the government thinks is acceptable. To put this in perspective, compare 5 tons soil erosion (10,000 pounds) to a 50-bushel soybean crop (60 pounds per bushel, 3000 pounds total) which is 3.33 pounds of soil lost for each pound of soybean harvested each year. The acceptable rate of 4-5 tons of erosion is still exceptionally high and not sustainable longterm.

Topsoil can erode due to strong winds, hard rains and flowing water. Farming practices like tillage leave the soil vulnerable to surface runoff. One way to help mitigate the loss of topsoil is to have farmers use no-till practices to grow crops. There are several types of no-till including strip till which is about 80% no-till. Only 6 inches strips are tilled where crops are planted. According to Dr. Richard Cruse, Iowa State University, "By and large, we have the technology now to make no-till work. However, it's more challenging with some soils than others."

According to USDA, no-till practices have already been implemented by 51 percent of USA farmers. Cover crops are only used on about 7.5% of USA farmland acres, but there has been nearly a 50% increase in cover crop adoption in the last 5 years. The use of no-till with cover crops or crops grown after the main crop is harvested protects the soil from moving off site by wind or water. Live roots anchor the soil and add soil organic matter. Plant leaves reduce rain velocity so that soil is not dislodged upon rain impact. A rain drop may dislodge soil particles up 1 foot high and out 3 feet. Live plants and roots also absorb fertilizer and soil nutrients and keep them from running off the soil.

When topsoil erodes, the nutrients crops need go with it, making it more difficult for soil to store water and support plant growth. On highly eroded soils, farmers can lose 50-70 of their potential yield. Rapid erosion is a problem because recovering topsoil is a slow process that takes years. Generating just over an inch of topsoil may take 1,000 years or more. Soil is regenerated when rocks in the subsoil break down into sand, silt, and clay. Soil organic matter accumulates slowly as roots and crop residue is decomposed by soil microbes.

Degraded soil makes growing food more difficult and expensive. Without healthy soil, farmers won't be able to grow nutrient-dense food to feed our growing population. In the last 20 years, the world population has increased from 6 billion to almost 8 billion people today. Some experts suspect that Earth could potentially run out of usable topsoil within 60 years if the high rate of soil erosion continues. Using no-till and cover crops, farmers may be able to reduce the rate of soil erosion down to a fraction of the current rate, in some cases as low as 100 to 300 pounds/acre/year. Zero soil erosion is not possible. The lower more natural rate of soil erosion is considered healthier for the environment.