Plant and Soil Responses to Biosolids–Phosphorus in Two Florida Soils with High Phosphorus Content

Abstract

Greenhouse studies were conducted to evaluate the effects of biosolids amendments on phosphorus (P) availability in two Florida pasture soils with medium to very high native P content. Three granular, anaerobically digested, heat-treated, exceptional quality biosolids (from Baltimore, MD, Tampa, FL, and Largo, FL), and a commercial fertilizer, concentrated super phosphate (CSP) were applied to A horizons of the two soils: the Millhopper sand [very high native P and oxalate extractable iron + aluminum (Fe + Al)] and the Candler sand (medium to high native P and oxalate extractable Fe + Al). After a four-month cropping season, bahiagrass (Paspalum notatum Flugge) yields did not vary with P-sources and application rates primarily because of the natively high soil-P. The effect of added P on P uptake by bahiagrass was minimal in the Millhopper soil, but significant in the Candler soil. The excessive native P and high P retention capacity of Millhopper soil masked the effect of P addition by the amendments. In the Candler soil, P uptake was much larger in treatments involving CSP and Largo biosolids, which have greater KCl-extractable (hence, readily available) P contents than Tampa and Baltimore materials. Biosolids-P availability varied in the order: Largo ≫ Tampa > Baltimore. The Mehlich I-P was better correlated with P uptake by bahiagrass in Candler soil than the Millhopper soil. Two factors appeared to control the availability of P in treated soils: the chemical forms of P in P-sources, and the chemical properties of soils. Native P contents and P fixing capacities of soils need to be considered while interpreting soil test results. This study demonstrated the presence of a soil P threshold beyond which there is virtually no effect of P addition on plant P uptake.