Soil Phosphorus Storage Capacity in Florida Soils Receiving Fertilizer Alternatives.

Global phosphorus (P) supplies are dwindling and thus there is a need to identify and develop alternative P fertilizer sources. Even so, P-impacted soils are also a concern and threaten water resources in some agricultural regions of the U.S., including south Florida. Slow-release, P fertilizer, recovered from municipal waste water treatment as biosolids, have been used in the past and new products, such as struvite, are being developed. Assessing their impact on land in Florida agricultural areas will help in the continued development of Best Management Practices (BMPs) and provide data for land and water managers tasked with minimizing P impacts in sensitive areas. A column experiment was conducted in greenhouse using two different agricultural soils (Spodosols and Ultisols). The aim of this study was to test Class B, Class AA, and biochar prepared from Class B biosolids as organic P sources, compared to inorganic P fertilizer sources (struvite and triple super phosphate) mixed with two soils with varying P retention and ortho-P release. A P loss risk assessment, based on a threshold P saturation ratio (PSR; a molar ratio of Mehlich-3 extractable P to [Fe+Al], beyond which P release increases sharply) was determined. The soil P storage capacity (SPSC) was calculated using the threshold PSR to assess potential environmental P loss risk. After 20 weeks, the columns were analyzed for Mehlich-3 P, Fe and Al, and water soluble P (WSP). The PSR and SPSC values were calculated. The WSP, an indicator of releasable P in soil following rainfall/irrigation events was: TSP>struvite>biosolids>control (unfertilized soil). Biochar prepared from Class B biosolids and Class AA biosolids released more P, compared to Class B biosolids from both Ultisols and Spodosols. Results suggest that biosolids and biochar derived from biosolids might make suitable P fertilizer alternatives, due to reduced short-term P availability in soils.