Treated wastewater solids called biosolids are sometimes used by farmers to boost soil nutrient levels. Now research by a U.S. Department of Agriculture (USDA) scientist has provided new information about how long those plant nutrients remain after biosolids have been applied to the soil.

The work, conducted by Agricultural Research Service (ARS) agronomist Eton Codling, revealed biosolids have the potential to boost soil phosphorous levels for years.

ARS researcher Codling measured mineral levels in three different soils that had received a single amendment from a biosolid processed either via high heat, additions of lime, anaerobic digestion, or air drying. The amendments, which were applied at several different rates to the soils, had taken place from 16 to 24 years earlier during previous studies on biosolids. As part of the earlier work, the fields had been cropped after the biosolids had been added, so the biosolid nutrients in the experimental fields had been available for crop uptake for at least 16 years before Codling began his research.

Codling observed that phosphorus levels were generally higher in the biosolid-amended soils than in soils that didn't receive the amendments. This strongly indicated that soluble phosphorus levels in biosolid-amended soils could exceed typical plant requirements for years after biosolids were added.

Codling, who works at the ARS Environmental Management and Byproduct Utilization Laboratory in Beltsville, Md., also noted that phosphorus solubility varied with the biosolid type and application level.

USDA assured biosolids used in agricultural production have been processed to kill pathogens, and their use is strictly regulated to ensure that the materials don't harm the environment, human health or animal health.