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Processes in the soil system

[…] Soils are complex, dynamic systems, in which an almost countless number of processes are taking place. Generally these processes can be classified as chemical, physical or biological but there are no sharp divisions between these three groups. For example, oxidation and reduction are usually regarded as chemical processes but they can be accomplished by microorganisms; similarly the translocation of min-eral particles can take place either in suspension or in the bodies of organisms such as earthworms.

Chemical processes

The main chemical processes include hydration, hydrolysis, solution, clay mineral formation, oxidation and reduction.

Hydration

Hydration is the process whereby substances absorb water. Few of the primary minerals undergo hydra-tion directly, therefore very little takes place during the early stages of weathering. The principal exception is biotite which absorbs water between its layers, expands, and finally splits apart. Hydration is more often a secondary process affecting decomposition products such as iron and aluminium oxides.

Hydrolysis

Hydrolysis is probably the most important process participating in the destruction of minerals and soil formation. It is the replacement of cations such as calcium, sodium and potassium in the structure of the primary silicates by hydrogen ions from the soil solution eventually leading to the complete decomposi-tion of the minerals. The products of hydrolysis such as calcium are then available to be taken up by plants or removed by water flowing through the soil or they may precipitate out of solution.

Solution

There are only a few substances found in soils that are soluble in water or carbonic acid. Nitrates, chlorides and sulphates are very soluble but these only occur in appreciable amounts in the soils of arid areas. Calcite and dolomite are less soluble but are widespread and form the major components of limestone, chalk and some other parent materials. These materials are very distinctive since they are almost complete-ly soluble in carbonic acid and therefore supply only a very small residue after solution. Consequently soils developed on these materials are normally quite shallow. Less soluble is apatite (calcium phosphate) which can persist for thousands of years in some soils of humid areas developed in drift deposits. On the other hand, most other materials, particularly the silicate rocks, furnish a considerable residue of primary and secondary products. Some minerals such as quartz which are usually considered to be inert and insoluble do dissolve eventually. This accounts for the small amount of primary material < 50 µm found in many of the very old soils of humid tropical and subtropical areas.

Products of hydrolysis and solution

These include the weathering solution, the resistant residue and alteration compounds. The weathering solution contains the basic cations together with some iron, aluminium and silicate ions which are partly or completely lost, redistributed in the soil system or taken up by plants. The resistant residue includes quartz, zircon, rutile and magnetite which alter only very slowly but do decompose when present as very small particles. The alteration compounds are principally hydroxides and oxides of iron and aluminium; silica and clay minerals. Iron forms ferric hydroxide , goethite -αFeO-OH or hematite - Fe2O3. Ferric hydroxide is an amorphous yellowish-brown substance that occurs in many soils in their initial stages of formation. Goe-thite is crystalline with reddish-brown colour but changes to yellowish-brown as it becomes hydrated. Goe-thite has a wide distribution, ranging from the tropics to the arctic and is one of the main colouring sub-stances in soils. Hematite is bright red and occurs chiefly in soils of tropical and subtropical areas or in old geological formations. The other iron oxide is Iepidocrocite which is bright orange in colour, occurring principally in soils subjected to periodic waterlogging. In a crystalline form, aluminium oxide occurs mainly in the soils of humid tropical and subtropical areas as gibbsite γAl(OH3) which is also the principal constituent of bauxite. Amorphous or hydrous silica, in addition to forming part of the clay, may be lost in the drainage water or redistributed within the soil system. Sometimes it may accumulate and cement the soil into a massive rock-like material known in Australia as silcrete. Finally there is manganese dioxide which is of restricted distribution being found as the blue black coating within the soil or associated with iron oxides and hydroxides in certain concretionary and massive deposits.

Transformation of individual minerals

The breakdown of the individual minerals depends largely upon the nature of the climate, thus the prod-ucts of decomposition of a given mineral will vary from place to place. For example, feldspars can be changed to mica in a cool climate, to kaolinite in a hot, moderately humid climate and to gibbsite in a very hot, very humid climate. This wide variability in the end products of weathering applies also to the amphi-boles and pyroxenes.

Clay minerals - their formation and properties

Early workers visualised a simple transformation from primary minerals such as feldspars to clay miner-als like kaolinite but it has been shown that the structures of these two minerals are so different that this is not possible. For many situations it seems that the primary minerals undergo fairly complete decomposi-tion to simple substances followed by the synthesis of the individual clay minerals. This involves princi-pally the rearrangement of the silicon-oxygen tetrahedra and the aluminium octahedra both of which be-come aligned to form sheets within the clay minerals. There are five main types of clay minerals important in soils; namely, kaolinite, montmorillonite,. hydrous mica, vermiculite and allophane. The first four are crystalline and can be regarded as small sheet-like particles (Fig. 8). Allophane is amorphous or very finely crystalline with an indeterminate composition but generally has about equal amounts of aluminium hydroxide and silica. One of the most important properties of clay minerals is that they have negative charges which allow them to adsorb and exchange cations on their surfaces i.e. they have a cation exchange capacity and because of variations in their structure they have widely differing cation exchange capacities. Also they have a capacity to absorb water. Kaolinite has a low cation exchange capacity (3-15 meq/100g) and expands very slightly when wet. On the other hand, the exchange capacities of vermiculite (100-150 meq/100g) and montmorillonite (80-150 meq/100g.) are high and both of these minerals can adsorb water and expand, particularly montmorillonite. Hydrous mica occupies an intermediate position with a moderate cation exchange capacity (10-14 meq/100g) and a small capacity for swelling when wet. The occurrence of these minerals in soils is related largely to pH. Kaolinite forms in acid soils with mica, vermiculite and mont-morillonite forming under progressively more alkaline conditions.

Flocculation and dispersion are two more properties of clays. Flocculation is the reaction whereby the individual particles of clay coagulate to form floccular aggregates. This can be demonstrated in the laboratory by adding a small amount of calcium hydroxide to a suspension of clay. Immediately floccular aggregates form and gradually settle to the bottom of the vessel - a similar reaction takes place in soils to form crumbs and granules. At the other extreme is the state of dispersion in which the individual parti-cles are kept separate one from the other by many ions particularly sodium. Thus depending upon the nature of the cations present in the soil it may either be in a flocculed or dispersed and often massive condition. Clay minerals scarcely ever occur in a pure form in soils; often a single particle is composed of interstratified layers of two or more different clay minerals, or they may be enmeshed in large quantities of oxides - hence the many difficulties encountered in their identification.

Oxidation and reduction

It is convenient to consider these two processes together since one is the reverse of the other. Iron is the principal substance affected by these processes for it is one of the few elements that is usually in the reduced state in the primary minerals. Consequently when it is released by hydrolysis and enters an aerobic atmosphere it is quickly oxidised to the ferric state and precipitates as ferric hydroxide to give yellow or brown colours. If, on the other hand the iron is released into an anaerobic environment it stays in the ferrous state. Such soils range in colour from blue to grey to olive to black depending upon the precise composition und that is formed. Vivianite (ferrous phosphate) imparts blue colours while black colours are due to sulphides which often form in coastal and estuarine marshes. The formation of horizons by partial or complete reduction is often referred to as gleying.

Physical processes

The main physical processes are translocation, aggregation, freezing and thawing and expansion and contraction, but the agencies responsible are very varied.

Aggregation

Aggregation is the process whereby a number of particles are held or bound together to form units of varying bur characteristic shapes.

Translocation

Many of the processes of soil formation and horizon differentiation are concerned primarily with re-moval, reorganisation and redistribution of material in the upper 2m or so of the earth's crust.

Percolating through the soil in a humid environment is a large volume of water which on moving down-wards takes with it dissolved material some of which may be translocated to a horizon below or it may be lost in the drainage water. On slopes, some moisture moves laterally causing the soils at lower positions to be enriched by soluble substances. Sometimes, the whole soil may become saturated by water moving laterally and there is free water at the surface. Such situations are known as flushes and usually carry a plant com-munity commonly indicative of a moist habitat. Fine particles and colloidal materials are often transport-ed in the suspension from one place to another within the soil system. Perhaps the most important manifestation of this process is the removal of particles < 0.5µm from the upper horizons of some soils followed by their deposition in the middle position to form cutans. Ultimately this can lead to differences of more 20 per cent in the clay content between adjacent horizons. As stated earlier many members of the mesofauna and some small mammals are responsible for redistributing large amounts of material within the soil.

Freezing and thawing

These two processes take place to varying degrees over a wide area of soiIs. During freezing a number of ice patterns develop as determined by a number of factors. Needle ice usually forms near to the surface of the ground and is responsible for heaving stones to the surface and the breakdown of large clods, hence the reason for ploughing before the onset of winter. Below the surface, freezing causes the formation of ice lenses and in polar areas repeated freezing and thawing causes profound disturbance of the soil and the development of a number of characteristic patterns such as stone polygons and mud polygons.

Freezing and thawing also causes rocks to be shattered leading to the development of extensive areas of angular rock fragments and a considerable amount of solifluction takes place in polar areas as a result.

Expansion and contraction

These processes take place mainly as a result of wetting and drying and are very important in soils contain-ing a high proportion of clay with an expanding lattice such as montmorillonite. They occur principally in the soils of hot environments with alternating wet and dry seasons. Contraction is probably the more important process for it leads to the formation of wide and deep cracks causing the roots of the vegetation to be stretched and broken. During expansion high pressures are developed within these soils, causing rupture and the slippage of one block over the other and the formation of polished faces or slickenslides on the blocks. Expansion and contraction also cause the formation of micro-topographic features known as gilgai. They can also cause the disruption of the foundations of buildings.

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