The availability of phosphorus in soils is controlled by the ability of plants to dissolve phosphate-bearing minerals, including apatite and feldspars. To satisfy the requirement of plants for phosphate, mineral dissolution competes with precipitation such as, for example, reactions involving lead or other heavy metals. Plants exude organic acid anions that very effectively enhance mineral dissolution but that may also liberate harmful solutes, such as aluminium. To make readily soluble chemical fertilisers, apatite in igneous and sedimentary rocks is mined and processed; in organic farming, phosphate-rich rocks are crushed and applied directly to the soil, relying on compounds produced by plant roots (exudates) to extract the phosphorus that plants need.

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