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Phosphate release in calcareous soils with elemental sulfur.
In many soils that are calcareous, a considerable amount of the phosphorus is contained as insoluble forms of calcium phosphate. As much as two-thirds of the total phosphorus in a calcareous soil may be in the form of insoluble calcium phosphate, which means it is unavailable for crop use. Insoluble calcium phosphates are the most abundant mineral form of phosphorus found, and are also the raw material from which most phosphate fertilizers are made. Insoluble phosphates are sometimes rendered soluble by chemical treatment with sulfuric acid.