Of the active soil formers, climate is perhaps the most important. Climatic elements involved in soil development are
(1) moisture conditions affecting the soil (precipitation, evaporation, and humidity),
(2) temperature, and
(3) wind.
Precipitation provides the soil water, without which chemical and biological activities ate not possible. When soluble chemicals are dissolved in water they ionize, or dissociate into positively and negatively charged particles. Without ionization the many complex chemical interchanges of elements necessary to soil development and plant growth cannot take place. An excess of precipitation, however, tends to leach away the colloids and ions. This process of downward migration of soil components by waters percolating through the soil is known as eluviation. A distinctive leached horizon of the soil, the A, horizon, results from this process. The deposition of colloids and bases in the underlying B horizon is a process known as illuviation.
In warm climates where rainfall is extremely heavy, silica (SiO2), is largely removed from the soil and carried off in streams. This process is termed desilication. Thus soils in the wet equatorial rain-forest belts are deficient in silica as well as in such bases as calcium, sodium, magnesium, and potassium, and are generally low in fertility.
In dry climates, evaporation exceeds precipitation and the soil is dry for long periods. Ground water is slowly brought to the surface by capillary attraction and evaporates in the soil, leaving behind the salts that were dissolved in the water. Calcium carbonate, the commonest of these deposits, forms a whitish crust, or hardpan, in the soil. In the southwestern United States this material is called caliche, and in places it makes the soil as hard and resistant to erosion as if it were a limestone. Gypsum (hydrous calcium sulphate) forms similar encrustations. In intermediate precipitation zones, such as the humid eastern border of the middle-latitude steppes, calcium carbonate appears as small nodules in the soil.
Rainfall and evaporation controls result in the formation of two major groups of soils.
(1) Pedalfer soils show pronounced leaching and occur in the eastern United States where the rainfall is more than 25 in (60 cm) annually,
(2) Pedocal soils have an excess of calcium carbonate and occur in the western United States where rainfall is less than 25 in (60 cm).
The names are coined from the chemical content of the soil. The syllables al and fer in pedalfer refer to aluminum and iron, respectively, and were chosen because excessive leaching leaves behind the aluminum and iron oxides as residual substances which thus become important in quantity. The syllable cal in pedocal refers, of course, to calcium, which is present in the carbonate form in all pedocal soils.
Temperature is another important climatic factor in soil formation. It acts in two ways.
(1) Chemical activity is generally increased by higher temperatures but reduced by cold, and it ceases when soil water is frozen. Thus, tropical soils have a parent material, which is thoroughly altered chemically, whereas soils of the frozen tundra have a parent material, which is composed largely of mechanically broken minerals.
(2) Bacterial activity is increased by warmer soil temperatures. Where bacteria thrive, as in the humid tropics, they consume all dead plants that lie upon the ground. Thus there is no layer of decomposing vegetation on the ground and little humus within the soils of the humid tropics. In cold continental climates, bacterial action is reduced, and a generous layer of decomposing vegetation (leaf mold) covers the ground under forests. Hence, raw humus is preserved at the soil surface and is important in the upper part of the mature soil profile.
Wind is of minor importance as a climatic factor in soil development. Winds may increase the evaporation from soil surfaces and may remove surface soil in arid regions lacking a plant cover. Windblown dust may accumulate and thereby provide the parent material of a soil.
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