Leaching behavior of phosphorus in sandy soils amended with organic material

Transport of phosphorus (P) from agriculture often causes eutrophication of surface water systems, which is particularly a concern in sandy soil regions because of low holding capacity of the soil for nutrients and moisture. Soil amendment is considered to be one of the best management practices in reducing P loss to water. A column leaching study was conducted to evaluate the effectiveness of organic amendment applied together with water-soluble fertilizer in reducing P leaching in two typical agricultural sandy soils (Spodosol and Alfisol) in south Florida. The Spodosol is an Alfic Alaquod, whereas the Alfisol is an Arenic Glossaqualf. Both soils have the same texture with sandy, siliceous, and hyperthermic properties. Organic material (OM) used in this study is marketed as a slow release organic fertilizer (GreenEdge; JEA Inc., Jacksonville, FL), which supplies essential nutrients for plant growth. The treatments consisted of chemical fertilizer (CF) alone, CF + 1.25 g kg(-1) OM, CF + 5.0 g kg(-1) OM, and CF + 10.0 g kg(-1) OM. A control receiving no CF or OM was also included. Leaching of orthophosphate-P (PO4-P) and dissolved total P was similar. After seven leaching events, application of CF together with 10.0 g kg(-1) OM reduced PO4-P and dissolved total P loss by 25.4% and 21.4% in the Spodosol and by 45.3% and 22.6% in the Alfisol and increased soil Olsen-P and Mehlich 3-extractable P by 54% and 31%, respectively, in the Spodosol and by 50% and 94% in the Alfisol, compared with CF alone. Increasing OM application rates increased NaHCO- and NaOH-extractable P but decreased water-extractable P in the two soils. Soil microbial biomass P (MBP) and the percentage of MBP in Olsen-P (MBP/Olsen-P) increased by 585% and 350%, respectively, in the leached Spodosol and by 190% and 127%, respectively, in, the leached Alfisol with the treatment of CF together with 10.0 g kg(-1) OM, compared with CF alone. It appears that the reduction in P leaching by OM amendment was related to the stimulation of microbial activities, which incorporate water-soluble P from applied CF into organic fractions and retained more P in soil against leaching but available to plants. These results indicate that organic amendment in combination with CF in a 10 g OM kg(-1) soil level can increase P utilization efficiency by reducing P leaching and maintaining a larger available P pool in sandy soils.