Soil erodes at a much faster rate than that at which it's created. Creation of productive soil is a major focus of organic gardening and sustainable agriculture. According to the Center for Earth Leadership, “in the last 40 years, 30 percent of the world’s arable land has become unproductive." An inch of topsoil can take 500 years to create, and it takes 6 inches of topsoil to grow food.
Over-grazing, detrimental agricultural practices and removal of vegetation are the primary causes of soil erosion. "We're losing more and more of it every day," said David Montgomery, a geologist at the University of Washington. "The estimate is that we are now losing about 1 percent of our topsoil every year to erosion; most of this caused by agriculture." Excessive tilling is now seen as one of the major causes of soil loss.
In the Dust Bowl of the crisis of the 1930s in the Great Plains, huge numbers of people lost their farms and livelihoods because of soil erosion. Years of plowing the land without replenishing it were followed by drought, and soon the land could not support life. Called desertification, this is now a problem on 30 percent of the earth’s land.
Soil subjected to continual tilling and the application of synthetic fertilizers without replacing organic content is the most likely to erode. These practices do not provide a vegetative cover for soil after crop harvest when it is most vulnerable to rain and wind. Sparse vegetative cover allows raindrops to penetrate the soil; it loosens and washes away. This soil needs protection.
Organic Matter Content
The organic matter content of soil determines its erodability. Soil particles cling to decaying plant vegetation on the surface and to roots underground. The no-till method of farming allows successive crop plantings to remain and decay after harvest, thus progressively adding organic content. Roots keep soil from eroding and the decay process adds living organisms and nutrients.
Farming practices that prevent erosion include drip irrigation, planting cover crops and green manure crops, contour cultivation and no-till methods. The Center for Earth Leadership reports that in one region of Africa where soil was eroded and degraded, the sorghum crop yield increased 400 percent when sustainable, non-erosion producing methods were introduced. Erosion can be prevented in home gardens by making and using compost to replenish the soil.