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Fresh green soybean plants with visible roots cutaway.
Cristie Preston, Ph.D.

In general, phosphorus is immobile in soil due to the chemical bonds a phosphate ion can form. Phosphorus from commercial fertilizer and soluble phosphorus in the soil — phosphate — is either present as H2PO4¯ or HPO4¯2 when the soil pH is near neutral. These are the forms of phosphorus that are plant available and can be taken up.

However, the oxygen that surrounds phosphorus in the phosphate ion allows for chemical bonds with other nutrients and soil clays depending on the soil pH. Phosphate readily binds with iron, aluminum, and manganese in soil solution and on iron/aluminum oxide surfaces when the pH is acidic (below 7), and phosphate binds with calcium or calcium carbonate surfaces when the pH is alkaline (above 7). The result is insoluble compounds. Therefore, phosphorus is rendered immobile and unavailable to plants.

Check out this related story on how soil pH affects phosphorus availability to determine optimum soil pH ranges for crop production.

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