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a With permission from Mason and Moore (1982).

a With permission from Mason and Moore (1982).

metals may exist in sparingly soluble or insoluble mineral forms, including occlusion in Fe or Mn oxides or as cations that have undergone isomorphous substitution in silicate clays. Generally, soluble and exchangeable pools of micronutrients represent a small proportion of the total, while the large majority remains in specifically adsorbed or bound pools. These pools are usually in equilibrium with each other such that if the micronutrient concentration is soil solution decreases due top plant uptake or leaching, insoluble forms may release the micronutrient.

Complexation of soluble metal ions with insoluble organic matter reduces bioavailability, whereas the formation of soluble organic complexes increases bioavailability. Plant roots and microorganisms in the associated rhizosphere produce and secrete large quantities of organic acids and other biochemical chelating agents (Stevenson, 1991). These chelating agents are able to form stable complexes with metal ions. These chelating compounds range from simple amino acids to humic and fulvic complexes and represent a significant proportion of the metal binding capacity of soils.

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