Figure 5.18 Cassava (Manihot esculenta) plants. Note the food-storage roots on the plant that has been dug up.
The soil became what it is today through the interaction of a number of factors: climate, parent material, topography of the area, vegetation, living organisms, and time. Because there are thousands of ways in which these factors may interact, there are many thousands of different soils. The solid portion of a soil consists of mineral matter and organic matter. Pore spaces, shared by variable amounts of water and air, occur between the solid particles. The smaller pores often contain water, and the larger
Figure S.1Q A soil profile. (Reproduced from the Marbut Memorial Slide Set, 1968, SSSA, by permission of Soil Sciences of America, Inc.)
ones usually contain air. The sizes of the pores and the connections between them largely determine how well the soil is aerated.
If one were to dig down 1 or 2 meters (3 to 6 feet) in an undisturbed area, a soil profile of three intergrading regions called horizons would probably be exposed (Fig. 5.19). The horizons show the soil in different stages of development, and the composition varies accordingly. The upper layer, usually extending down 10 to 20 centimeters (4 to 8 inches),
is called the topsoil. It is usually subdivided into a darker upper portion called the A horizon and a lighter lower portion called the E horizon. The A portion contains more organic matter than the layers below.
The next 0.3 to 0.6 meter (1 to 2 feet) is called the B horizon, or subsoil. It usually contains more clay and is lighter in color than the topsoil. The C horizon at the bottom may vary from about 10 centimeters (4 inches) to several meters (6 to 10 feet or more) in depth; it may even be absent. It is commonly referred to as the soil parent material and extends down to bedrock.
The first step in the development of soil is the formation of parent material from existing rocks that have not yet been broken down into smaller fragments. Parent material accumulates through the weathering of three types of rock, which originate from various sources. These sources and types include volcanic activity (igneous rocks); fragments deposited by glaciers, water, or wind (sedimentary rocks); or changes in igneous or sedimentary rocks brought about by great pressures, heat, or both (metamorphic rocks).
Climate varies greatly throughout the globe, and its role in the weathering of rocks varies correspondingly. In desert areas, for example, there is little weathering by rain, and soils are poorly developed. In areas of moderate rainfall, however, well-developed soils are common. In some areas of high rainfall, the excessive flow of water through the soil may leach out important minerals. Similar leaching out of important minerals may occur when garden sprinklers are left on all night. Many gardeners and house-plant enthusiasts have stunted or killed the very plants they were trying to foster by "drowning" them with too much water or too frequent watering. As a general rule, plants should not be watered unless the soil surface feels dry.
In areas where there are great temperature ranges, rocks may split or crack as their outer surfaces expand or contract at different rates from the material beneath the surface. When water in rock crevices freezes, it expands and causes further cracks and splits.
There are many kinds of organisms in the soil, as well as roots and other parts of plants. In the upper 30 centimeters (1 foot) of a good agricultural soil, living organisms constitute about one-thousandth of the total weight of the soil. This may not sound significant, but it amounts to approximately 6.73 metric tons per hectare (3 tons per acre). Bacteria and fungi present in the soil decompose all types of organic matter, which accumulates when leaves fall and roots and animals die. (This process is further discussed under the section on composting in Chapter 17.) Roots and all other living organisms produce carbon dioxide, which combines with water and forms an acid, thereby increasing the rate at which minerals dissolve. Ants and other insects, earthworms, burrowing animals, and birds all alter the soil through their activities and add to its organic increment either through wastes that they deposit or through the decomposition of their bodies when they die. Humus, which consists of partially decomposed organic matter, gives some soils a dark color.
The total organic composition of a soil varies greatly. An average topsoil might consist of about 25% air, 25% water, 48% minerals, and 2% organic matter. Soils in low, wet areas, where a lack of oxygen keeps microorganisms from their normal activities, may contain as much as 90% organic matter. Except in legumes and a few other plants, almost all of the nitrogen utilized by growing plants, as well as much of the phosphorus and sulfur, comes from decomposing organic matter. In addition, as organic matter breaks down, it produces acids, which, in turn, decompose minerals. Other roles of organic matter in the soil are discussed in the section on soil structure.
If the topography (surface features) is steep, soil may wash away or erode through the action of wind, water, and ice as soon as it is weathered from the parent material. It has been estimated that more than 20 metric tons of topsoil per hectare (8.2 tons per acre) is washed away annually from some prime croplands in the central United States.
If an area is flat and poorly drained, pools and ponds may appear in slight depressions when it rains. If these bodies of water cannot drain quickly, the activities of organisms in the soil are interrupted, and the development of the soil is arrested. The ideal topography for the development of soil is one that permits drainage without erosion.
Soil Texture and Mineral Composition
Soil texture refers to the relative proportions of sand, silt, and clay in a given soil (Table 5.1).
Sands are usually composed of many small particles bound together chemically or by a cementing matrix material. Silt consists of particles that are mostly too small to be seen without a lens or a microscope.
Clay particles are so tiny that they can't be seen with even a powerful light microscope, although they can be seen with an electron microscope. Individual clay particles are called micelles. Micelles are somewhat sheetlike, negatively charged, and held together by chemical bonds. The negative charges attract, exchange, or retain positively charged ions. Many of the positively charged ions, such as magnesium (M++) and potassium (K+), which are needed for normal plant growth, are absorbed with water by the roots. Clay is
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