“Snirt” is not another trendy buzzword. It is the mixture of snow and dirt that represents a real and growing ecological problem. Visible in wintry rural ditches, snirt embodies the twin problems of soil erosion and water pollution. Its effect is as close as the tap water you drink and as far reaching as the growing dead zone in the Gulf of Mexico.
Snirt is formed when winds blow across bare farm fields each winter. As the dirty snow washes downstream with spring rains, the movement of soil from fields to waterways damages both the farms and the waterways.
Soil erosion is the first part of the equation, with wind and water carrying away fertile topsoil, the nutrient-rich upper layer of earth that plants need to grow. Some places have just a few inches of topsoil, while others luxuriate in meters-deep layers. An inch of topsoil takes centuries to form but can be lost within a few years. “While soil is technically a renewable resource, its slow rate of formation makes it practically irreplaceable,” according to the U.N. Food and Agriculture Organization.
In fertile Iowa fields, for example, topsoil measured 14 to 16 inches deep in the 19th century. By 2000, only 6 to 8 inches remained, and erosion continues. A 2007 U.S. Department of Agriculture (USDA) report estimates soil erosion from cultivated land at 5.7 tons per acre for Iowa and 5.9 tons per acre for Minnesota. Both states have registered marked decreases in wind and water erosion since 1982, but both still show unacceptably high rates of erosion. The USDA estimates that natural processes can regenerate five tons of topsoil per acre each year. Some scientists say the USDA’s assessment significantly underestimates erosion and greatly overestimates the soil’s capacity to regenerate.
Loss of fertile soil degrades the land’s capacity to produce. As topsoil erodes, fields lose nutrients, soil changes structurally, and it becomes less able to hold water. More artificial fertilizers must be applied to ensure continuing crop yields. That raises the cost of growing a crop and increases the danger of chemical pollution of waterways and aquifers. The fertile soil stripped from farm fields transforms into sediment, clogging streams, narrowing rivers and affecting the health of waterways and fisheries.
Snirt and other wind and water erosion contribute to the pollution of our water resources, sending sediment and nitrates into streams and waterways. The washing of excess nitrogen from farm fields when fertilizer is applied in the spring and early summer causes some nitrate pollution. More nitrate pollution happens after harvest and before spring planting. That’s when soil, with no living plants to hold it in place, releases nitrogen that washes downstream. For example, nutrient pollution is the cause of dead zones, including the more than 5,000-square-mile dead zone that forms in the Gulf of Mexico each spring.
Nitrates flow down through the earth as well as downstream on the Mississippi to the Gulf. Aquifers provide drinking water for half the U.S. population, and nitrates readily contaminate shallow groundwater. Des Moines, Iowa, recently sued surrounding counties to stop the runoff that’s putting nitrates in Des Moines’ drinking water. “Too much nitrogen, as nitrate, in drinking water can be harmful to young infants or young livestock,” according to U.S. Geological Survey.
Soil erosion is hard to see. Snirt makes it visible, if only briefly, offering clear evidence of erosion. The more serious effects such as groundwater pollution and sedimentation of rivers do not show up so well in photographs.
Solutions to soil erosion exist and have been known for decades, if not centuries. They include:
- Windbreaks: planting trees between fields so the wind is not as strong and carries away less soil.
- Crop rotation: alternating row crops such as corn (which uses lots of nitrogen), soybeans and other legumes (which bind nitrogen into the soil), and nonrow crops such as alfalfa, which has deep root systems to aerate, fertilize and improve soil.
- Cover crops: crops planted after the harvest of row crops and left in place all winter. Their roots, stems and leaves greatly reduce soil erosion.
- Contour planting: planting across a hillside, rather than up and down a slope, to reduce runoff and erosion. Of course, the bigger solution is to plant permacrops rather than row crops on steeper hillsides and to take marginal land out of production.