Healthy topsoil has between 100 million and 1 billion microorganisms per teaspoonful, according to the USDA Natural Resources Conservation Service, and it is these organisms that create the nutrients to feed plant life. Topsoil is a community of life that includes beneficial bacteria, fungi, organic matter, rock dust and particles, plant roots and lichen. Good soil makes a good garden. Topsoil erosion severely reduces the nutrient content of soil, and therefore of the nutritional value of food grown on it.
Topsoil is the uppermost layer of the earth where plants are grown for food and get their nutrient content. It is the top 8 to 12 inches of the earth and the EPA describes it as "the most productive soil layer." It takes 500 years for natural processes to create 1 inch of topsoil, which is why erosion is a problem. Loss of topsoil means loss of the nutrients that grow food. Topsoil erosion also causes loss of the structure that holds roots in place.
Causes of Erosion
Agricultural practices that result in topsoil erosion include exclusive use of row cropping and plow-tilling; inappropriate cultivation of slope land; and the use of tilling without replacing lost organic matter into the soil. Repeated mechanical tilling changes the structure of the soil so it erodes more quickly. Row crops that are harvested without leaving adequate plant litter behind leave the soil unprotected and vulnerable to erosion.
Damage Caused by Erosion
Erosion causes organic matter and its nutrients to wash away, giving plant life no sustainable source for growth. Synthetic fertilizers have traditionally been used to replace nutrients lost to erosion, but their use is now understood to be a contributing factor in global pollution problems. Patricia Muir, plant ecologist at Oregon State University, reports that "about half of the fertilizer applied to U.S. farmland is necessary just to replace nutrients that are lost with soil erosion."
All measures taken to reduce excessive topsoil erosion also decrease the need for water and synthetic fertilizer. Farm conservation tillage systems are designed to preserve topsoil by leaving at least 30 percent of crop residue on the ground after harvest. Topsoil conservation can easily be practiced in the home garden by refraining from "turning the soil over" and by the continual addition of organic matter (compost). The organic matter decays slowly, bringing billions of microorganisms to the topsoil where they can provide nutrients for growing plants.
Improvement of topsoil health is a major focus of organic gardening techniques. The National Sustainable Agriculture Information Service provides farmers and home gardeners with in-depth information about soil conservation (please see Resource). Its Conservation Stewardship Program rewards farmers and ranchers for current conservation practices and for putting in place additional new sustainable practices.