Regardless of the different properties of their layers, all soils contain the same five components: mineral particles, decaying organic matter (humus), air, water, and living organisms. Differing amounts of these materials define the soil's properties and, therefore, the plants it can support. Just as there is no such thing as a typical cell, there is no typical soil. Indeed, there are more than 70,000 kinds of soils in the United States alone based on bedrock, organic material, and other characteristics.
Weathering breaks rocks into progressively smaller pieces, the smallest of which are called soil particles. These particles consist of minerals, which are naturally occurring inorganic compounds that are used by plants as building blocks. All soils contain three kinds of soil particles: sand, silt, and clay. Sand particles are the largest (0.02-2 millimeters), clay particles are the final product of weathering and the smallest of these particles (< 0.002 millimeters), and silt particles are between sand and clay particles in size. Different kinds of soils contain different proportions of sand, silt, and clay. Soils having a mixture of sand, silt, and clay—the soils in which most plants grow best—are called loams. Sandy soils are usually nutrient deficient and poorly suited for growing crops.
The amount of water held in a soil is proportional to the surface area of its particles: the larger the surface area, the greater the retention of water. Clay particles are smaller than sand and therefore have a larger surface area per unit of soil volume than does sand (because there are so many more clay particles per unit volume). Indeed, the surface of clay particles in the upper few centimeters of soil in a 2-hectare cornfield equals the surface area of North America. As a result, clay soils retain much more water than do sandy soils. This property of clays would seem to make them ideal for plant growth. However, this is not entirely true because the small size of clay particles results in their packing tightly together—so tightly that the clay has low amounts of oxygen, due to small air spaces. This tight packing also retards the penetration of water into the soil (e.g., water penetrates clay about 20 times slower than it penetrates sand). As a result, much of the water that falls on clay soil runs off and is unavailable for plant growth. This runoff water also contributes to erosion. Tightly packed clay also can impede root growth.
Humus is the decomposing organic matter in soil. The amount of humus in soil varies, usually from 1% to 30%. The most extreme examples are the soils of swamps and bogs, which may contain more than 90% humus. These soils are usually so acidic that decomposers can hardly grow in them. As a result, humus accumulates faster than it is broken down in these watery habitats.
The amount of humus in soil affects the soil and its plants in several ways. The lightweight and spongy texture of humus increases the water-retention capacity of the soil. Water absorption by humus decreases runoff, thereby slowing erosion. Humus swells and shrinks as it absorbs water and later dries. This periodic swelling and shrinking aerates the soil. Humus is also a reservoir of nutrients for plants. Like time-release vitamins, humus gradually releases nutrients as it is degraded by decomposers. Humus also provides habitats for many organisms that mix and concentrate nutrients. Most plants grow best in soil containing 10% to 20% humus.
Because humus is constantly being decomposed by bacteria and fungi, it must be replenished. In natural ecosystems, humus is replenished by leaves that fall off plants and the addition of animal wastes. Many gardeners replenish humus in soil by composting, a method of converting organic wastes to fertilizer. The process is simple: organic wastes (e.g., lawn clippings) are gathered into a compost pile, where they are degraded by fungi and bacteria. During winter, this decomposition may take several months, while during summer, it may be completed in as little as 3 weeks. The resulting compost costs less than commercial fertilizer, provides an efficient means of disposing of waste products, and contains nearly all of the nutrients needed by plants. Moreover, composting releases nutrients gradually as the materials decompose, thereby providing plants with a continuous balance of nutrients. On a larger scale, farmers often plow under the stubble of plants remaining after a harvest to maintain the humus in soil used to grow crops.
About 25% to 50% of the volume of most soils is air. This "empty space" has a critical role: it is the conduit by which oxygen reaches the roots. When water replaces a soil's air for a long period, such as in flooded soil or overwatered house-plants, roots can become anaerobic and die. Clay soils, with their tightly packed clay particles, contain less air than do sandy soils. Thus, aeration of a clay soil can be improved by adding silt, sand, or organic material, which increases the penetration of water into the soil, thereby stimulating root growth.
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