soil development

soil development To produce the three-dimensional organization of a well-developed soil requires the relatively consistent application of a complex suite of physical, chemical, and biological processes (see soils). The fact that basic sets of soils all over the world contain recognizable horizons implies that some processes are common to the development of all soils. As soils are open systems, these processes can be grouped into those that result in inputs, outputs, transfers, and transformations. The main inputs or additions to the upper parts of soils are organic matter and the contained elements from surface vegetation and water from precipitation; this water can also contain dissolved elements. There may be an additional input of particulate matter to the soil by wind and water action and by mass movement processes, such as soil creep. Energy input is provided by incoming solar radiation, and the combined temperature and moisture status determines the nature and rate of operation of most soil processes. Weathering of bedrock can also be considered as an addition or input, although some workers regard it mostly as a transformation. Outputs from the soil system include the downslope movement of soil particles and water, throughflow, deep percolation, leaching, and the uptake of water and nutrients by roots. Leaching is the downward movement of water through the soil which results in the removal of water-soluble minerals.

Soil development is mainly the result of transfers and transformations. Transformations include the great range of organic compounds that form during the decomposition of organic matter and the weathering of primary and secondary minerals. Transformation of energy can also occur. Transfers are probably the most important group of processes; they include the redistribution of energy and the internal reorganization of matter. There is the translocation of iron, clay, humus, and hydrated ions; ion exchange; and the diffusion of gases. Eluviation is the movement of soil material through the soil zone, resulting in depletion in certain horizons and accumulation in others. Illuviation is the precipitation or accumulation of material leached from upper horizons within what is known as the B horizon (which is usually in the middle of the soil sequence). Material can also move up the soil profile, for example by capillary rise of dissolved ions and mixing by soil fauna. In areas subject to freeze–thaw activity, frost heave (cryoturbation) will create a similar effect.

As a soil develops, distinctive horizons become apparent. These are essentially created by processes of transfer and transformation, as can be illustrated by examining a number of specific examples. Decalcification (Fig. 1a) is the result of the leaching of calcium, but the process may be balanced by the release of calcium by the weathering of bedrock. If the production of nutrients by the weathering of bedrock is small, and if the rate at which cations are replaced from vegetable litter is poor, the effect of leaching may not be balanced. Under such conditions the stability of soil aggregates will decrease and clays will break down and deflocculate. Lessivage (Fig. 1b) then occurs; this is the movement of silicate clays in colloidal suspension within the soil profile without any change in chemical composition. The soils that are then produced are known as acid brown earths, sols lessivés, or luvisols in the FAO–UNESCO classification (see soils).

Podzolization (Fig. 1c) occurs in freely drained soils on siliceous parent materials, usually under a cool, temperate climate where the vegetation provides a litter deficient in bases that promotes the formation of raw acid organic horizons. Under such conditions base cations such as Na⁺ and Ca⁺ are leached rapidly and are not replaced by other processes; in consequence, clay minerals break down and hydrous oxides of iron and aluminium are mobilized and removed from the upper horizons. This results in the bleached eluvial horizon characteristic of podzols. Some of this mobilized material accumulates lower down in the profile to produce a diagnostic horizon enriched in aluminium and iron. The iron can become indurated to form an iron pan.

Waterlogging in the soil, which can be the result of a perched water table or high groundwater levels, will alter the balance between oxidative and reducing conditions (Fig. 1d, e), creating a mottled effect in the zone where the water table fluctuates. The mottling is created by the alternation of ferrous and ferric (oxidized) iron compounds. In the humid tropics, where intense weathering is occasioned by high temperatures and high water availability, the process of ferralitization produces a red soil as a result of the dehydration of iron compounds (Fig. 1f); this reddenning is known as rubefaction. During this process, iron and aluminium compounds are removed from upper horizons and redeposited lower down in the soil profile.

In semi-arid environments the amount of precipitation may be insufficient to produce complete leaching of solutes. Sodium and potassium may be removed but calcium is often precipitated as calcium carbonate, a process known as calcification (Fig. 1g). Under more arid conditions only the more mobile ions of sodium and potassium are dissolved. These, however, are soon precipitated and may also be drawn upwards by intense surface evaporation to be precipitated near the surface. This is the process of salinization, which produces a group of soils known as solonchaks.

Soil development is not necessarily a gradual sequential process. A change of climate or of vegetation cover will affect the processes and may lead to a complex soil with one type superimposed on another. Excessive erosion of soil and excessive burying (known as retardant upbuilding) may lead to a regression of soil development. Soil is a complex entity and its development is consequently complicated.

John Gerrard

Bibliography

FitzPatrick, E. A. (1980) Soils: their formation, classification and distribution. Longman, Harlow.
Ross, S. (1989) Soil processes: a systematic approach. Routledge, London.