Implications for agriculture and soil pollution

The speciation of metal cations governs their availability to plants and their potential to contaminate waterways (Bernhard et al., 1986). Available forms of metal cations are not necessarily associated with one particular chemical species or a specific soil component. Hence, in order to predict the availability of metal cations, either we have to establish the species involved and develop methods that determine specifically those forms only, or we have to establish an empirical relationship between an accepted diagnostic measure of the metal and plant growth. In reality, a combination of these two approaches produces the most accurate predictions with the least amount of effort. 

Speciation of trace elements may vary with time, depending on the solid-phase components that are present, pH, and the number and accessibility of adsorption 

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be 1 106 before 50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

For nutrient cations, the formation of metal complexes can increase their uptake by increasing the total concentration of the metal in solution, which, in 

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