In terms of importance to human populations, the most significant aspect of weathering is in the development of soil. Soil can be defined as weathered material that will support the growth of rooted plants.
A heap of quartz sand is not soil according to this definition, for it will not support vegetation. Nor is the so-called “lunar soil” of the moon a true soil. It is merely disintegrated mineral matter and should more properly be termed regolith. Regolith refers to the debris of rock and mineral matter that rests on solid bedrock. It includes soil, alluvium, and weathered fragments of the bedrock.
Soil is not a mere accumulation of particles derived from mechanical and chemical weathering. It is a vital natural material that would not develop were it not for the activities of a myriad of bacteria, fungi, worms, and insects.
Actually, soil can be considered an intricate mixture building materials and damage valuable works of art. Presently, millions of dollars are being expanded in acid rain research in the hope that a solution to the problem can be found. Aside from local chemical treatment of particular areas, however, a long-term solution must involve reduction in atmospheric pollutants that form acid rain.
ADVERTISEMENTS:
Factors Governing Soil Development
Five factors are involved in soil formation. These are climate, parent material, topography, time, and biologic activity. Of these factors, climate is of greatest importance.
Indeed, soil types and climate are so closely related that maps showing global distribution of climates resemble maps showing the occurrence of different soil types. Temperature and rainfall, of course, influence not only rates of weathering but the nature of vegetation.
Given enough time, soil-informing processes operating within a given climate will prevail over differences in parent material and provide a basically climate-controlled soil type. The reserve is also true. Identical rocks in temperate high-rainfall regions and in semitropical arid regions will produce quite different soils.
ADVERTISEMENTS:
The influence of parent material on the formation of soils is of secondary importance but can be observed in soils that have recently developed from unaltered bedrock.
Such soils stills retain textural and mineral components that are derived from the parent rock but which may become obscured after the soil has evolved to a further stage. In some cases, various influences of parent material will persist in the developing soil. Soils developed above quartz sandstones may tend to be more acidic than soils formed on limestones, and these differences will be reflected by the natural vegetation living on each soil type.
Soils forming on rocks that are deficient in certain elements such as magnesium or boron will ordinarily also lack these constituents. Not all soils are developed from underlying consolidated bedrock. Many fine agricultural soils are formed on loosely consolidated sediment laid down by streams, winds, glaciers, or the waters of lakes.
Topography also influences the ultimate nature of soils. On steep slopes, for example, erosion is usually more vigorous and the soil layer generally thinner. Also, water falling on slopes is partially lost to runoff, and therefore there is less water available for percolation into deeper layers of soil. Even the direction of slope may have a bearing on soil development. In the northern hemisphere, slopes facing toward the south tend to be warmer and dryer and therefore support quite different kinds of plants than do the northern slopes.
ADVERTISEMENTS:
The factor of time in soil formation is often not fully appreciated by those who clear away soil for construction or mining and expect nature to quickly restore this important resource. The development of soil is an exceedingly slow process.
Depending on climatic conditions, several hundred to several thousands of years are required to produce a fertile soil. Agricultural soils formed during this lengthy natural process may be lost to erosion in only a few years, particularly in areas where soil conservation measures are not followed. The problem is not trivial, for an estimated 3 billion metric tons of good agricultural soil is lost each year from agricultural fields in the United States alone.
Soil Horizons
Soils are not uniform in texture and composition from bedrock to the surface but rather consist of a number of fairly distinct a years or soil horizons, which differ in composition and physical characteristics. The stack of soil horizons is called a soil profile. A simple profile, such as might develop in a humid region on granitic rock, has three distinct divisions. Soil scientists have named these layers the A, B, and C horizons.
ADVERTISEMENTS:
The A horizon, also known as “top soil,” which lies immediately beneath the surface, is characterized by a high content of organic matter. It is also a zone in which soluble compounds are dissolved and, along with fine clay particles, carried downward to be deposited in the underlying B horizon. Because of these losses, the A horizon is referred to as leached or eluviated, whereas the B layer is sometimes called the washed in or illuviated zone. Iron and clay minerals tend to accumulate in the B layer.
In humid regions one can often recognize the B horizon by its more brownish colour and clayey texture. The C horizon of the soil profile consists of partially altered parent material. In it one finds evidence of chemical weathering, but soil development has not progressed to the level at which the original characteristics of the bedrock are unrecognizable. Beneath the C horizon lies unaltered parent material.
Soils developed on limestones in humid regions often do not exhibit a clear division between the B and C horizons. In such soils, the A horizon consists of dark humus-rich material that changes downward into relatively high coloured clay and then stained and partly dissolved limestone. The clays in these soils are residues of clay impurities left when the limestone was dissolved.
How does the sequence of soil horizons develop in a humid region underlain by granitic rock? Imagine that glacination has recently stripped away all loose sediment over a granite outcrop. Initially, disintegration and decomposition of the granite will produce a sandy layer composed mostly of grains of quartz the feldspar along with clay derived from the weathering of silicate minerals.
ADVERTISEMENTS:
As the layer of granular material thickens, plants establish themselves, and organic material from a variety of sources becomes incorporated into the surface layer. Meanwhile, with each rainfall, soluble ions and clay particles are flushed downward from the surface layer into a lower level. This lower level soon takes on the clayey character of the B horizon. While these events are taking place, solutions reaching still deeper continue the chemical attack on the parent material, thus perpetuating the C horizon.