Why Are There Many Kinds of Soils?
Soil is one of the principal substrata of life on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem. It has evolved through weathering processes driven by biological, climatic, geologic, and topographic influences.
The basic material of the surface of the earth is solid rock, and the surface of the landscape we see is nearly always the result of weathering, the action of sun, wind, frost, rain, ice or snow. Therefore, there are many kinds of soil depending upon the climate and the type of parent rock. There is also a third factor which influences the kind of soil formed. This is the vegetation.
The first step in weathering is the breaking down of the rock. Water plays an important part at this stage, either by freezing and shattering the rock as it expands or by washing away some of the minerals of which the rock is composed, thereby loosening its particles.
The climate is also important for most rocks contain much quartz as well as silicates. In a cold climate, the crystals of the silicates are dissolved more quickly than the quartz. In a hot moist climate however, the quartz is washed away and the silicates left behind. Every intermediate stage can be found between these two extreme types of soil.
Vegetation also plays its part by splitting rocks with its roots. Also dead branches and leaves fall on to the ground, decay and add a layer of humus to the soil, rich in nutrients which enable larger plants to grow. These larger plants support animals, some of which help to mix the soil still further.
There are other types of soil such as those formed from the silt deposited by rivers—alluvial soils—or where bogs and marshes occur. But one thing is common to all soils. They are very unstable if the vegetation which covers them and aids in their formation is removed. Then they are easily washed away and the result of centuries of slow development is lost.
As stated at the beginning of this article, soils evolve under the action of biological, climatic, geologic, and topographic influences. The evolution of soils and their properties is called soil formation, and pedologists have identified five fundamental soil formation processes that influence soil properties. These five “state factors” are parent material, topography, climate, organisms, and time.
Since the rise of agriculture and forestry in the 8th millennium BCE, there has also arisen by necessity a practical awareness of soils and their management. In the 18th and 19th centuries the Industrial Revolution brought increasing pressure on soil to produce raw materials demanded by commerce, while the development of quantitative science offered new opportunities for improved soil management.
The study of soil as a separate scientific discipline began about the same time with systematic investigations of substances that enhance plant growth. This initial inquiry has expanded to an understanding of soils as complex, dynamic, biogeochemical systems that are vital to the life cycles of terrestrial vegetation and soil-inhabiting organisms—and by extension to the human race as well.