Nearly all the changes which occur during pedogenesis require liquid water: in dry or frozen soils the rate of weathering or pedogenesis is negligible. In general, water moves vertically up or down in a soil carrying with it (leaching) the soluble or colloidal products of weathering. Some lateral movement does occur but the overall trend of movements is downwards. The matrix constituents of soils are not all equally soluble, quartz can be regarded as essentially insoluble.
The profile development process is therefore marked by differential movement of constituents: some, such as salts of potassium or sodium may be leached completely
Elements may be trapped in those soil fractions formed from compounds of elements where IP ranges from 30 to 95. This is a group of elements which tend to accumulate in weathering residues and includes the elements Cr(III), Fe(III), Mn(III), Mo(IV), V(III) and V(V). Iron and manganese dominate the group and in most soils these elements precipitate as hydrous oxides either in concretionary forms or as coatings on mineral and clay surfaces. They exert a chemical control on the activities of other ions far greater than might be supposed from their concentrations (Jenne, 1968). The hydrous oxides act as sinks for other trace elements through several mechanisms (Hem, 1977; Loganathaneio/., 1977;Okazaki etal., 1986). As they form, other trace metals can be occluded in the oxide precipitate and cobalt is notable for its association with manganese oxides. They tend to have high adsorption affinities which increase with pH and the trace cations are readily sorbed onto the oxide surfaces after which they may enter the precipitate through solid state diffusion. Superimposed on these mechanisms is the possibility that the hydrous oxides can dissolve and then precipitate again in response to changes in the soil's redox potential or pH.
Two important chemical parameters control the nature of the leaching regime. The first is acidity, ie, pH. Desert and semidesert soils may have pH values up to about 10 because of the presence of sodium bicarbonate in the soil solution but usually a pH value of about 8.3 represents the upper limit. Soils with pH values in the range 7.0-8.3 are those formed on limestone and the soil solution is dominated by carbonate/bicarbonate. Many metal carbonates have low solubilities and leaching losses are therefore minimal. With increasing acidity leaching losses become more pronounced and at pH = ca. 4.0 the clay minerals start to degrade and aluminium dominates the soil solution. Agricultural soils are usually maintained at pH values above 6 and enhanced solubility of metal ions under the more extreme acid conditions is not important. However, in recent years concern has mounted over the effects of 'acid rain', ie, increased wet and dry deposition of sulphates or nitrates from industrial sources. Berggren et al. (1990) have reported that forest soils in southern Scandinavia have become, on average, 0.5-1.0 pH units lower during the course of recent decades. The solubility of Al, major cations, Mn, Cd and Zn has increased and large areas of forest soils now have acidity conditions such that any additional input of strong mineral acids causes a rapid increase in the release rates of these elements.
The second control is the reduction-oxidation potential, Eh. In most soils the dissolved oxygen content of the soil solution is high enough to ensure that oxidising conditions prevail. Where water percolates slowly through the profile or where water is stagnant in the soil, reducing conditions are found and the process of gleisation occurs. Sulphate reduces to sulphide and most metal sulphides are very insoluble. For agricultural soils marked reductions in soil Eh do not occur but the control of redox potential on metal solubility is an important consideration for padi rice and in wetland soils which may be drained and aerated during reclamation. De Laune et al. (1981) maintained sediment suspensions at pH values of 5.0, 6.5 and 8.0 and Eh values of - 200 mV, 0 mV, 250 mV and 500 mV. Iron and Mn concentrations decreased significantly as pH and Eh were raised. Calmano et al. (1986) have reported how changes from neutral, reducing environments to moderately acid, oxic environments lead to increased bioavailability of Cd.
Weathering and leaching are not the only processes that cause the development of horizons. Plants draw their moisture and nutrients from a relatively large volume of soil but, assuming they are not harvested, will return their inorganic constituents to the surface through leaf fall or generally to the upper layers through root death. The fresh organic material is food for many organisms, which, in turn, live on each other. Thus, organic material changes until the end product, a black or brown organic material with a lignin-like structure, called humus which enriches the surface layers. Humus is a store of trace metals through adsorption, since it carries a negative charge like the clays, or through complexation (chelation). Metals may be released as humus oxidises (mineralises) or by desorption or decomplexation (Shuman, 1988; Livens, 1991).
Was this article helpful?