A Publication of Texas Farm Bureau Agriculture in the Classroom

What is Soil?

How Is Soil Formed?
Soil is the loose top layer of the earth's surface in which plant life grows and on which we walk. Soil is made of many things. Minerals, pieces of rock, living organisms, bits and pieces of decaying plants and animals, water, and air can all be found in different amounts, depending on the soil type.

Soil is formed over many years. Soil originates from parent material, or rock, which is slowly ground into smaller particles by friction, temperature changes, freezing water, and chemical action. When rocks grind against each other small particles are scraped off. This can happen as rocks tumble around in a stream of water. Rocks found in a river or stream are usually smooth and rounded because of this grinding action. Glaciers that moved across a big part of the United States many years ago ground up huge quantities of rock. Much of the soil in the north central United States was formed by the action of these glaciers.

Changes in temperature also help to make soil. Rocks warm up from the sun during the day and expand. At night the rocks cool and contract. This expansion and contraction chips away at the rock, reducing it into smaller and smaller particles. When water finds its way into cracks in the rock and freezes, it expands, exerting tremendous force. This breaks the rock into smaller pieces.

Plant roots give off carbon dioxide gas which combines with water in the soil to form an acid. This acid dissolves certain kinds of rock, particularly limestone and marble, leaving small particles that are not dissolved by the acid. This is a chemical action that helps form soil.

What Is Soil Made Of?
Another component of soil is organic matter or humus. This is material that was once living, such as plants, animals, and insects. As these die, they are acted on by other organisms in the soil such as bacteria, fungi, and earthworms. The material decomposes, and is mixed with the soil to provide nutrients for other living organisms. The fertility of a particular soil is largely determined by the amount of organic matter it contains.

Soil also contains air and water. Between the soil particles are tiny spaces. The size of these spaces is determined by the size of the soil particles. Generally, the larger the particle size, the larger the pore spaces. These spaces can be filled with air or water and nutrients that plants need for growth. Most plants require that a soil drains well. This means that the pore spaces are large enough that water can move through them easily. If the pore spaces are too small, as in clay, plants will drown because the soil holds water too long. If the pore spaces are too large, as in sand, water drains out of the soil too fast. If the pore spaces are the right size, water can move through the soil, both downward and upward. This movement is called capillary action. Due to capillary action plant roots can use water that is below them as well as water nearer the surface.

Soils are usually comprised of a combination of three different sized particles: sand, the largest; silt; and clay, the smallest. Sand particles are gritty and can be seen with the naked eye. Silt particles feel smooth like flour. Clay particles cannot be seen with the naked eye and, when wet, feel sticky to the touch.

Types of Soils
Soils can be classified according to the percentage of each size particle they contain. This is done by determining the soil texture. Soils can be classified as sand, sandy clay, sandy loam, loam, loamy sand, clay loam, etc.

The soil type determines what kind of crop will do best. For example, rice prefers a clay soil; one that holds water well. Peanuts prefer a sandy soil; one that drains well and allows the peanut, which grows underground, to develop. Most agricultural crops such as cotton, corn, wheat, and grain sorghum prefer a well drained soil. A well drained soil is one that holds enough water in its pore spaces to be available to the plant roots until the next rain, but not so much that water stands at the surface for an extended time.

Importance of Soil
We depend on the soil to grow our food and much of the fiber that we use for clothing. Erosion can cause a farmer or rancher to lose his most valuable asset: his soil. Erosion is the movement of soil from one place to another by wind or water. When the wind blows across bare, unprotected soil it picks up soil particles and blows them away. As the wind becomes stronger it can pick up larger and larger particles, finally leaving only hard-packed subsoil that is unproductive for growing plants. The soil that is blown away is deposited in other areas, sometimes miles away, and can actually become so deep that it smothers other plants. During the Dust Bowl of the 1920s many farmers lost their farms because the soil either blew away or was covered by a layer of sand. This was caused by the land being plowed and left without a protective cover of plants.

Water also causes soil to move from one place to another. When heavy rains fall on sloping, unprotected soil, the water moves downhill, picking up soil particles as it goes. Gullies are cut in the land with the soil particles being deposited at the lower end of the field, or in a stream or river, where they are carried even further away. If this continues the land will become too rough to cultivate and the most productive soil will be lost.

Farmers and ranchers have learned how to protect their soil from wind and winter erosion. One way is to plant a cover crop which protects the soil from being washed or blown away. Another practice farmers use is contour plowing, which means instead of plowing up and down the slope, they plow at right angles to the slope. Farmers also construct terraces that divert the water off the field slowly, in the direction they want it to go. When crops are harvested the stubble is left to provide cover for the soil. Steeply sloped land is not farmed, but instead planted to grass for grazing livestock. The grass keeps the soil covered. This prevents erosion, while at the same time allowing the farmer or rancher to raise cattle, sheep, goats, and other livestock that can convert the grass into meat, milk, and other products used by humans.

In areas where wind erosion is a problem farmers plant wind breaks. These are rows of trees on the side of the field where the strongest winds come from. The trees help slow the speed of the wind so it can't pick up soil particles. Wind erosion can also be controlled by keeping the soil covered with plants during the time of year when the wind is most likely to blow.

Soil erosion has been significantly reduced since the 20s and 30s. It still occurs, but agricultural producers have learned how to control and minimize it. The United States Department of Agriculture Natural Resources Conservation Service (NRCS) is a federal agency that provides technical assistance to farmers and ranchers helping them control erosion on their land. The NRCS has offices in nearly every county in Texas and provides educational information to educators.

The Texas Soil and Water Conservation Board is a state agency that also assists farmers and ranchers in soil and water conservation.

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