What is the formation of soil? Information on the physical and chemical changes, effects of plants and factor of soil formation.
ORIGIN AND FORMATION OF SOILS
For many hundred million years, until life started, the earth had no true soil—only air, water, and rock. Great geological processes were at work long before the plants appeared, however. Mountains were thrust up several times in the geological past and then worn down by weathering and erosion. Changes from heat to cold and the action of ice broke the rocks into small bits. Some parts were carried deeply into other rocks or to the ocean, where they settled out. The sediments were in turn pressed into new kinds of rock and again thrust up into mountains. Volcanos also spread out new soil-forming material, most of the soils from relatively fresh volcanic ash being quite fertile. This accounts for many of the fertile soils of Indonesia and other lands near young volcanos.
Physical and Chemical Changes.
At several times great continental glaciers crept down over northern and central Europe and over northern North America. The last great advance ended only 10,000 years ago. The masses of ice plucked rocks from the hills they passed over and ground them into fine clay, sand, and pebbles. Part of this mixture was left on the land as glacial till by the melting ice, and a part was carried out by glacial streams and later deposited. Even now the work of smaller glaciers can be seen in the high mountains or wherever there is much snow and cold.
Sometimes fine particles of broken rock were picked up and carried by the wind. In this way, great mantles of silty material, or loess, were deposited in parts of the Mississippi Valley, large parts of eastern and central Europe, China, and many other places. From the deserts the fine material blew away to leave a cover of small stones. The sand was deposited in dunes near the margins. This same kind of blowing on a reduced scale can be seen today during extreme drought and where there is no* protective cover of vegetation.
Along with these processes of physical weathering go the slower chemical changes in the rock material. These chemical changes are slowest where it is cold and dry, and fastest where it is hot and moist. In the tropics there are soil materials with chemical compositions very different from those of the mother rock. They tend to accumulate iron and aluminum, while other common mineral elements—even silicon— nearly disappear in time. These rock-destroying and earth-forming processes go on continually, some of them very gradually, others with catastrophic violence.
The Effects of Plants.
With the coming of plants and animals, true soil formation began. The green mantle of plants greatly reduced erosion, breaking the force of rains and allowing more water to soak into the earth and deep underground spaces and less to run off the surface. The plants also acted to stabilize the soil against the force of the wind. The first soils were little more than unconsolidated fine rock-mere mixtures of clay, silt, and sand, with small pebbles and huge boulders sometimes mixed in. However, plant and other life forms brought great changes to the land. The plants sent roots down, brought up water and nutrients, took in carbon dioxide from the air, and synthesized food with the energy of sunlight. This food built and nourished the plants, and the fallen leaves and dead parts became food for small animals, fungi, and bacteria in the soil. As they consumed the organic matter, part of the minerals and nitrogen in it was released to the soil for other plants, part was sometimes leached down or out of the soil with long rains, and part was used by these microorganisms themselves to build other organic matter—which, in turn, formed soil. Soils continue to be formed today by the same processes.
Humus, the more stable part of the decomposing organic matter, builds up in the soil until the processes of its decomposition just balance its accumulation. The total amount of organic matter in a soil at this equilibrium point is very different in unlike soils, depending on the plants, the climate, the kind of mineral material, and the surface features in a given area. Thus, in a natural soil, there is a continuous cycle of nutrients from the whole soil into the plant and back to the soil again, and plants in all soils tend to concentrate both the organic matter and the nutrients in the surface.
However, other processes tend to oppose this concentration. Leaching tends to wash materials down into the lower layers. Some plants produce kinds of organic matter that make the iron soluble enough to move down. More calcium and magnesium, for example, can be leached from the upper layers of soil than the plants are then able to return to the soil. Where leaching is heavy and the plants are weak “feeders” on the basic elements, the upper part of the soil—or even the whole.of it—becomes very acid. On the other hand, where adjacent plants feed very heavily on calcium, the soil may be neutral, not acid. A few trees in the humid tropics feed heavily on sulfur from both the air and soil. The effect of this process is to make the surface extremely acid.
Yet again, dusts and fine particles falling out of the air, as well as gases adsorbed by the falling rain or rpoist soil, help to counteract the effects of leaching. Over the centuries, dust and other materials from the air permitted trees to grow on soils from extremely nutrient-poor rocks under dense tropical rain forests. The surface soil becomes well charged with nutrients, the cycle from soil to plant and back to soil being so nearly complete that nutrients added from the air and rain balance those lost by leaching.
However, if the forest is cleared and the soil cropped, it loses its fertility in five years or less, and it takes many years for the forest to return. Also, although most of the additions from the air are beneficial, some others, like the salt from ocean spray, or the fumes from industrial plants, are not.
Factors in Soil Formation.
Each natural kind of soil is the product of a unique combination of five factors: (1) living matter (plants, microorganisms, and animals); (2) climate (heat, cold, rain, snow, and wind, and their cyclic distribution between night and day and through the seasons); (3) parent rock materials (fineness and mineralogical and chemical composition); (4) relief (landform and slope); and (5) time. The effect of any one of these factors depends upon the effects of the others.
Living matter and climate are the active factors in soil formation. Differences in these account for greatly different soils from similar glacial till in areas between Maine and Montana, for example, or from similar granites in New York or the basin of the Congo River. The parent rock is a passive factor—the stuff on which first the destructive processes of weathering and then the constructional processes of soil formation operate, although both sets of processes actually overlap a great deal. Relief is a conditioning factor, modifying the rainfall by allowing rapid runolf on steep slopes or ponding in low places, and modifying the temperature by differences in the angle at which the sun’s rays strike the soil.
Thus, in northern latitudes, soils on south-facing slopes are warmer and drier than those on north-facing ones. In places where the soil is kept so moist by a combination of rainfall and relief that waterlogging prevents decomposition of organic matter as it forms, peat accumulates. With slow decomposition it darkens, so that a nearly black organic soil may lie beside a light-colored mineral soil. And finally, it takes time for these processes to produce the characteristics that different soils have—a short time for some features and thousands of years for others.
On a local scale, the effects of parent rock, relief, and age can cause the soil in one garden or field to be quite unlike that in the adjoining one. However, the differences among the broad groups of soil found across a continent are related primarily to differences in vegetation and climate.