Topsoil is the uppermost 8 to 10 inches of soil. It is called the "A horizon" in soil classification systems. All the world's food resources are dependent on topsoil's nutrient content and its ability to regenerate. The worldwide loss of topsoil through erosion and detrimental agricultural practices is a primary cause of global desertification problems. Topsoil is regenerated in home gardens through the regular addition of compost.
The USDA soil taxonomy classification system describes horizon "A" soil as the earth's layer in which the most biological activity takes place. Soil organisms such as earthworms, arthropods, nematodes and fungi and many species of bacteria create the "A" horizon's life. It is home to plant root systems and is the source of nutrients for food crops. Topsoil is continually created through the decomposition of plant and animal life.
The earth's topsoil erosion currently exceeds topsoil formation. Topsoil is formed at the rate of 1 inch per 500 years. "As a result of erosion in the last 40 years, 30 percent of the world's arable land has become unproductive," according to the earth advocacy group The Center for Earth Leadership. The primary causes of topsoil degradation are overgrazing, deforestation, removal of vegetation for development, and poor agricultural practices.
The use of chemical fertilizers and pesticides contributes to topsoil degradation by killing the small organisms in the soil that create and sustain its life. Topsoil is a complex living food web that contains millions of essential bacteria, fungus, protozoa, worms and nematodes per teaspoonful. Poisoning the topsoil food web with chemical fertilizer destroys soil viability within 40 to 50 years, according to NRCS soil scientist Dr. Elaine Ingham.
Topsoil Food Web
Topsoil health is defined by its ability to cycle nutrients, control water flow, and provide natural habitats for plants and animals. When topsoil is amended with nitrogen-based fertilizers its microorganisms do not reproduce and soil becomes lifeless. Bacteria, fungus, worms, nematodes and other minute forms of animal life create topsoil's structure, its ability to hold water, transmute soil toxins and produce soil nutrients.
Topsoil preservation is an important focus of global efforts in sustainable agriculture. Solutions to erosion include using drip irrigation methods, planting cover crops as green manure, using organic fertilizer and practicing contour and no-till methods of cultivation. A recent experiment in Burkina Faso, Africa, using compost to reclaim degraded land resulted in a 400 percent sorghum crop yield, according to the Center for Earth Leadership.