Introduction

Soil, the thin layer of unconsolidated mineral and organic materials at the earth’s surface not covered by water, has been and continues to be seriously degraded by erosion by water and/or wind at many sites throughout the world. Soil is the natural medium for plant growth, and humans obtain much of their food, fiber, and various other materials from plants. Plants also are the main source of food (e.g., forages, grains, seeds, tubers, and roots) for animals, birds, and other creatures; they serve as shelter for animals and provide for an aesthetic environment. Conservation of soil, therefore, is highly important.