Water or air that moves over a soil surface applies forces to the soil particles on the surface. If the forces are large enough, they move the particles. The stronger the current or wind, the more particles are put into motion. Larger and heavier particles tend to roll or hop near the soil surface. Finer and lighter particles are carried upward from the soil surface. In nature, the finest particles may be carried for hundreds or even thousands of kilometers high in the atmosphere before they fall out!
Knowing how water passes through soil is very important. For example, engineers need to know how quickly water will drain away from buildings or bridges. Drainage rates are also important to farmers, landscapers, gardeners, environmental scientists, and other professionals. Water is one chemical found in soil. There are many others. Some of the chemicals in soil are very important for plant growth. Farmers and gardeners often test their soil for these chemicals using soil-testing kits. These kits can be bought at gardening or hardware stores.
Soil scientists separate soil using a stack of several sieves. The sieves have holes with slightly different sizes. The coarsest sieve is at the top of the stack, and the finest sieve is at the bottom. Most soils contain many kinds of material. All soils consist mainly of two kinds of material: particles of minerals and rocks, and organic matter. Organic matter is any matter that is or once was living.
There are many kinds of soil. A group of soil scientists from the U.S. made up a way of grouping soils that is used around the world. This grouping has hundreds of named soil types! All soils, however, are made of just a few main things. Soil consists of fine particles of minerals and rocks, decaying plants, and living plants and animals. You can easily see the larger plants and animals. There are even more tiny plants and animals that you can only see with a microscope.
Soil forms as the solid rock of the Earth, called bedrock, breaks down. It usually takes thousands of years for soil to form from bedrock. In some places, soil forms directly on top of bedrock. In other places, soil forms on a thick layer of loose rock and mineral material. This material, called sediment, has been carried from distant areas by rivers or glaciers.