Topsoil is the upper surface of the Earth's crust, and usually is no deeper than approximately eight inches (20 centimeters). The Earth's topsoil mixes rich humus with minerals and composted material, resulting in a nutritious substrate for plants and trees. It may one of the Earth's most vital resources, because it represents a delicate nutritional balance that provides food for many of the animals on Earth, either directly in the form of plant material or indirectly in the form of products from animals that eat plants.

The delicate nature of topsoil was imperfectly understood until very recently. Traditionally, farmers have rotated crops because they discovered that rotation created a better crop yield, but they did not understand why this was. During the Dust Bowl of the 1930s, farmers planted profitable crops over and over again, rather than rotating, and ultimately stripped the topsoil of nutritive value. The stripped soil did not support plants, which hold topsoil to the earth, which meant that even light winds could pick up the limited remaining topsoil and transport it elsewhere.

Modern sustainable farming practices place a heavy emphasis on crop rotation to conserve valuable topsoil. Farmers keep crops in rotation, allow fields to lie fallow, and plant nitrogen fixing plants like beans to promote soil health. Many farmers also plow plant material into the topsoil to enrich the humus, and spread compost and manure on it to make the topsoil more nutritious and rich. Healthy topsoil is dark brown, moist, and crumbly, with chunks of plant material and rich earth. Unhealthy topsoil is pale gray, thin, and uniform in texture.

Farmers who do not use environmentally sensitive practices are slowly killing off their topsoil. In some areas of Africa, Australia, and the Midwest, farmers are working with only a few inches of topsoil and a growing pharmacopoeia of fertilizers. As the topsoil becomes more and more degraded, more complex formulations are needed in an attempt to restore the balance of nutrients to the soil. This exhausted soil can take years to repair with organic practices after it has been abandoned by conventional farmers. The ultimate result if these farming techniques continue is desertification, a process which is already being observed in Africa and Australia.

Farmers are concerned about topsoil because it sustains their crops, but biologists also keep an eye on global topsoil health. Plants need healthy topsoil to survive, and it is actually very easy to damage. In nature, an assortment of plants combine with natural cycles of fire and water to maintain the topsoil. Plants hold the topsoil to the Earth, water nourishes the plants, and the plants die or are burned in fires to restore nutrients to the topsoil. When this cycle is disrupted by farming or clearcutting, the results can be disastrous.

In addition to being damaged by exhaustive farming practices, topsoil can also be lost through heavy water run off. Run off over clearcut land complicates matters, because there are no trees and plants to hold the precious topsoil down. As a result, major storms in heavily logged areas drag tons of topsoil off the Earth into the watershed, where it clogs rivers and makes them uninhabitable for fish. When the rivers reach the ocean, the sudden excess of nutrients causes fish die offs which sometimes stretch for miles out to sea beyond the mouth of the river.