SOIL CONSERVATION

Soil is essential for the growth of plants, which in turn provide food for animals and human beings. Soil consists chiefly of minerals mixed with organic (plant and animal) matter. Soil forms from rocks and similar materials that are broken up into smaller particles by physical and chemical processes called weathering. The particles become mixed with humus, a substance formed from plant and animal remains. Bacteria in the soil break down the humus into nutrients needed by plants.

The thin layer of fertile soil that covers much of the land was formed by natural processes over thousands of years. But in many areas, careless human practices have destroyed the soil in just a few years.

Rain, wind, and other natural forces gradually wear away the soil. This process, called erosion, normally occurs slowly. But people have greatly increased the rate of soil erosion by removing natural vegetation to clear land for construction projects, mines, or farmland. Plants protect soil from rain and wind. Their roots form an underground network that holds soil in place. Plants also absorb some rain water so that less runs off the land. Thus, fewer soil particles are washed away.

Soil erosion has long been a major conservation problem, especially on croplands. In the United States, soil erosion has severely damaged millions of acres or hectares of land. Much of the soil eroded each year ends up in lakes, streams, and rivers.

Farmers can reduce soil erosion by planting trees and leaving patches of natural vegetation between their fields and on other unplowed areas. The trees serve as windbreaks, and the plant cover slows the runoff of rain water. Many farmers also practice such soil conservation methods as contour plowing, strip cropping, terracing, and minimum tillage.

Contour plowing is practiced on sloping land. Farmers plow across a slope, instead of up and down. The plowed soil forms ridges across the slope. The ridges help slow the flow of rain water.

Strip cropping also helps slow the flow of rain water down a slope. Farmers plant grass, clover, or other close-growing plants in strips between bands of corn, wheat, or other grain crops. Grass and clover hold water and protect the soil better than grain crops do.

Terracing helps prevent soil erosion on hillsides. Farmers build wide, flat rows called terraces on the hillsides. A terraced hillside resembles a large staircase. The terraces hold rain water and so prevent it from washing down the hillside and forming gullies.

Minimum tillage, also called conservation tillage, consists of several methods of reducing the number of times a field must be tilled. Normally, farmers till their fields three or more times each growing season. One form of minimum tillage is called zero-tillage or no-till. After harvesting a crop, farmers leave the residues (remains) from the crop on the field as a covering for the soil, instead of plowing them under. During the next planting, the farmer prepares the seedbed with a device that leaves the residues between the crop rows. Zero-tillage not only provides cover for the soil but also conserves tractor fuel.

Another major conservation problem on farmlands is declining soil fertility, which is caused partly by planting the same crop in a field year after year. Corn, wheat, and other grain crops drain the soil of an essential chemical called nitrogen if they are grown on the same field for several years. Farmers can maintain the fertility of the soil by practicing crop rotation, in which crops are alternated from year to year. The rotation crop is usually a legume, such as alfalfa or soybeans. Unlike corn and wheat, legumes restore nitrogen to the soil.

Some farmers add plant remains or manure (animal wastes) to their fields to enrich the soil. Many use chemical fertilizers for this purpose. However, excessive use of some chemical fertilizers may decrease the ability of bacteria to decay humus and produce nutrients naturally. As a result, the soil may gradually harden and lose much of its ability to absorb rain water. The soil then erodes more easily. In addition, the chemicals from fertilizers may wash out of the soil and enter lakes, streams, and even wells, polluting the water. Excessive use of pesticides causes similar problems.

A common problem on irrigated farmland is the build-up of various chemical salts in the soil. Most irrigation water contains small amounts of these salts. In time, the salts accumulate in the soil and may reduce plant growth and ruin cropland.

SOIL CONSERVATION

The soils of farmlands, grazing lands, and forestlands provide many products and recreational areas. Soil conservationists work to ensure the wise use of these soils.

Wise use of farmlands involves maintaining a high level of nutrients and organic matter in cultivated soils. Farmers add organic matter to the soil by plowing under certain green plants. They also add fertilizers and rotate crops to replace nutrients that leaching and growing plants remove. In addition, farmers plow and plant their fields in ways that control erosion.

Grazing lands that have been overgrazed also suffer from erosion. Overgrazing decreases the amounts of plant life and organic matter in the soil, and the soil erodes easily. Ranchers conserve grazing lands by limiting the time that their herds graze in one area.

Forestlands also must be protected from erosion. In some cases, foresters leave unusable branches and other parts of trees on the forest floor to add organic matter to the soil. They also develop large groups of trees whose roots protect the soil by holding it in place against wind and water erosion.

CHARACTERISTICS OF SOIL

The method and rate of soil formation differs throughout a body of soil. As a result, the soil develops layers. These layers are called soil horizons. Soil horizons may be thick or thin, and they may resemble or differ from the surrounding horizons. The boundaries between the layers can be distinct or barely noticeable.

Most soils include three major horizons. The upper two, called the A and B horizons, are the most highly developed layers. The A horizon is also known as topsoil. The lowest horizon, called the C horizon or the subsoil, is exposed to little weathering. Its composition resembles that of the parent material. Pedologists describe soils by the characteristics of the soil horizons, including (1) color, (2) texture, (3) structure, and (4) chemical conditions.

Color. Soils range in color from yellow and red to dark brown and black. The color of a soil helps pedologists estimate the amounts of air, water, organic matter, and certain elements in the soil. For example, a red color may indicate that iron compounds are present in the soil.

Texture of a soil depends on the size of its mineral particles. Sands are the largest particles. The individual grains can be seen and felt. Silts are just large enough to be seen, and clays are microscopic. Pedologists divide soils into textural classes according to the amounts of sand, silt, and clay in a soil. For example, the mineral portions of soils classified as loam contain from 7 to 27 per cent clay and less than 52 per cent sand. In silty clay, more than 40 per cent of the mineral particles are clay, and more than 40 per cent are silt. Texture helps determine how thoroughly water drains from a soil. Sands promote drainage better than clays.

Structure. When soil particles aggregate, they form clumps of soil that are called peds. Most peds range from less than 1/2 to 6 inches (1.3 to 15 centimeters) in diameter. Their shape and arrangement determine a soil's structure. The ability of peds and soil particles to stick together and hold their shape is called consistence.

Most soils contain two or more kinds of structures. Some soils have no definite structure. In some such soils, the peds lack a definite shape or arrangement. In others, the particles do not aggregate.

There are three main kinds of soil structures: (1) platelike, (2) prismlike, and (3) blocklike. Platelike peds are thin, horizontal plates that occur in any horizon. Prismlike peds are column-shaped subsoil structures. Blocklike peds look like blocks with flat or curved sides. Large, flat-sided, blocklike peds commonly occur in subsoils. Small, rounded, blocklike peds make up most topsoils. They contain more organic matter and hold water and nutrients better than do larger peds.

Chemical conditions. Soils can be acid, alkaline, or neutral. The amounts of acid and alkali in a soil influence the biological and chemical processes that take place there. Highly acid or alkaline soils can harm many plants. Neutral soils support most of the biological and chemical processes, including the process by which green plants obtain many nutrients. This process is called cation exchange. Many nutrients and other elements dissolve in the soil solution, forming positively charged particles called cations. The negatively charged clay and humus attract some cations and prevent them from being leached (washed away) from the topsoil by drainage waters. The solution that remains in the soil contains other cations. Nutrient cations on the clay and humus and those in the soil solution change places with nonnutrient cations that are on roots. The roots can then absorb the nutrients.

HOW SOIL IS FORMED

Soil begins to form when environmental forces break down rocks and similar materials that lie on or near the earth's surface. Pedologists call the resulting matter parent material. As soil develops through the centuries, organic material collects, and the soil resembles the parent material less and less. Glaciers, rivers, wind, and other environmental forces may move parent material and soil from one area to another.

Soils are constantly being formed and destroyed. Some processes, such as wind and water erosion, may quickly destroy soils that took thousands of years to form.

Soil formation differs according to the effects of various environmental factors. These factors include (1) kinds of parent material, (2) climate, (3) land surface features, (4) plants and animals, and (5) time.

Kinds of parent material. The type of parent material helps determine the kinds of mineral particles in a soil. A process called weathering breaks down parent material into mineral particles. There are two kinds of weathering, physical disintegration and chemical decomposition. Physical disintegration is caused by ice, rain, and other forces. They wear down rocks into smaller particles that have the same composition as the parent material. Sand and silt result from physical disintegration.

Chemical decomposition mainly affects rocks that are easily weathered. In this kind of weathering, the rock's chemical structure breaks down, as when water dissolves certain minerals in a rock. Chemical decomposition results in elements and in chemical compounds and elements that differ from the parent material. Some of these substances dissolve in the soil solution and become available as plant nutrients. Others recombine and form clay particles or other new minerals.

The mineral content of parent material also affects the kinds of plants that grow in a soil. For example, some plants, including azaleas and rhododendrons, grow best in acid soils that contain large amounts of iron.

Effects of climate. Climate affects the amount of biological and chemical activity in a soil, including the kinds and rates of weathering. For example, physical disintegration is the main form of weathering in cool, dry climates. Higher temperatures and humidity encourage chemical decomposition as well as disintegration. In addition, decaying and most other soil activities require warm, moist conditions. These activities slow down or even stop in cold weather. Therefore, soils in cool, dry climates tend to be shallower and less developed than those in warm, humid regions.

Effects of land surface features also influence the amount of soil development in an area. For example, water running off the land erodes the soil and exposes new rock to weathering. Also, soils on slopes erode more rapidly than those on flat areas. They generally have less time to form and therefore develop less than do soils on flat terrains.

Effects of plants and animals. Soil organisms and organic material help soil develop, and they also protect it from erosion. The death and decay of plants and animals add organic material to the soil. This organic material helps the soil support new organisms. Soils that have a cover of vegetation and contain large amounts of organic material are not easily eroded.

Effects of time. Soils that are exposed to intense soil formation processes for long periods of time become deep and well developed. Soils that erode quickly or have been protected from such processes for a long time are much less developed.