Minimizing phosphorus leaching from a sandy clay loam caused by phosphorus fertilizers

At moderate to high fertilization rates, sandy-textured soils can leach much phosphorus (P) threatening surface water quality. High rates are used to compensate for P leaching, but there is also potential to reduce P leaching by using different P fertilizers. We examined the effect of poultry manure (PM), sheep manure (SM), triple superphosphate (TSP), sewage sludge of Sanandaj (SSS), sewage sludge of Toyserkan (SST), and biochars of Sanandaj and Toyserkan sewage sludges (BSSS and BSST, respectively) applied at a rate of 100 mg P kg^-1 (equivalent to 220 kg P ha^-1 yr^-1, the current regional practice for capital applications designed to raise and maintain soil P in the region) on P leaching over 10 pore volumes (equivalent to 589 mm rainfall) through a sandy clay loam soil widespread in Iran (and the Middle East). Phosphorus leaching losses decreased in the following order: TSP > SM > PM > SST > BSSS > control > SSS > BSST. The leachability of fertilized soil was best estimated by measurement of the mobile KCl-P fraction. At the capital application rate used, SSs or their biochars represented the least risk of P leaching and could be used in place of highly soluble manures or TSP to either protect water quality or maintain more P in the soil. However, this should only occur after confirming that this substitution does not impair agronomic performance.