Phosphorus binding

by Christopher B. Lind

Field trials of the commercial application of alum have been conducted in poultry houses over several years and involved millions of birds. These trials have demonstrated continuously the reduction of soluble phosphorus of up to 84%. In liquid waste applications, such as swine waste and cattle feed lot runoff, the reduction in soluble phosphorus has exceeded 95%.

The best phosphorus-binding chemical produces a precipitate that makes the P insoluble over a large pH range. Aluminum phosphate is very insoluble from pH 2 to >9. Neither aluminum nor phosphorus is returned to the environment under normal soil or water conditions.

Iron suitably binds soluble P, and its presence in a soil or sediment is largely responsible for much of the “background” P binding. Ferric phosphate is also very insoluble, about one order of magnitude more soluble than aluminum phosphate. Still, it is very effective. Under anoxic conditions, the ferric phosphate reduces to ferrous phosphate, which is soluble, thereby returning the phosphorus to the water or soil as a nutrient. The oxygen content of the soil or water does not effect aluminum compounds.

Calcium binds phosphate compounds best at high pH (>10). At pH values normally found in soil and water (<8.5) they are soluble and used as fertilizers and feed additives. Calcium phosphate compounds are several orders of magnitude more soluble than iron and aluminum phosphates and are not efficient in binding phosphorus.