Designer Composts May Combat Phosphorus Overload

Applying manure and composts as nitrogen fertilizer often adds more phosphorus than plants need. This extra phosphorus can then leach or run off into water. But specially formulated composts can make the phosphorus less soluble in water, thus minimizing the chance that it will wash away.

ARS agronomists Rufus Chaney and Eton Codling have been searching for inexpensive ways to make phosphorus less water soluble, or to increase the ability of manure, biosolids and composts to hold onto the phosphorus.

Chaney and Codling, with the ARS Animal Manure and Byproducts Laboratory in Beltsville, Maryland, found that composts high in iron could markedly help the manure and compost retain phosphorus. Both iron and aluminum oxides increase adsorption of phosphorus. These can be added as chemical additives or by mixing byproducts rich in iron or aluminum with the manure or other feedstocks before composting.

Because phosphorus runoff can damage streams, lakes, rivers and other waterways, limits on soil phosphorus are being proposed. Many states have implemented manure management regulations aimed at preserving groundwater quality and the health of major water sources.

A management tool called the Phosphorus Index (PI) is used to assess the risk of phosphorus loss from agricultural fields to surface waters. In some states, the PI is based on plant-available phosphorus, but the scientists found that adding iron and aluminum oxides to manure or composts reduced the water solubility of phosphorus much more effectively than the PI test indicates.

The findings should help livestock producers limit phosphorus runoff a time when they face tougher restrictions on this valuable soil nutrient.

ARS is the U.S. Department of Agriculture's chief scientific research agency.