Phosphate sorption and release in a sandy-loam soil as influenced by fertilizer sources

Soil phosphate (PO4-P) sorption and release is affected by reactions at the solution-soil surface interface. The objective of this study was to determine how the net negative surface charge and phosphate adsorption/desorption processes in a sandy-loam soil were affected by cattle manure and inorganic fertilizer applications. The soil came from a field experiment where manure and triple superphosphate (TSP) were applied annually on an equivalent plant-available P basis. After 4 yr, manured soils had 37% more negative surface charge in soil suspensions from pH 4 to 7 than TSP-fertilized soils. Phosphate adsorption was 23% lower in soils receiving manure than TSP The phosphate adsorption data (adsorption of added phosphate + native adsorbed phosphate) was modeled with the Langmuir 2-surface equation, which revealed that the binding strength for phosphate, including native adsorbed phosphate, was about 50% less in the manured soils than the TSP-fertilized soils. These results suggest that manure applications that increase the net negative surface charge also reduce the soil phosphate adsorption capacity. The cumulative desorbed phosphate was twofold greater from manured soils than TSP-fertilized soils. This confirmed the results from phosphate adsorption isotherms showing that phosphate was weakly bound in manured soils, compared with TSP-fertilized soils. We found that manuring not only decreased phosphate retention by the soil but also increased phosphate release into the soil solution, and thus may lead to greater transport of phosphate from agricultural land to waterways.