Accumulation of Phosphorus In An Organic Farming System Following Seven Years of Organic Amendments and Conservation Practices.

Application of animal waste to agricultural soil has resulted in accumulation of phosphorus (P) in soil with potential export of the soluble P into surface waters. We investigated accumulation of P in an organic farming system at the George Washington Carver Agricultural Experiment Station at Tuskegee University. The experimental design was a randomized-complete-block design with four replications and four treatments. Each year late in fall, crimson clover inoculated with Rhizobium was planted (34 kg ha^-1) with exception of the control. In spring, the cover crop was cut and left on the field and commercial NPK fertilizers (NPK-treatment) were applied as triple super phosphate (180 kg P₂O₅ ha^-1),  potassium chloride (120 kg K₂O ha^-1), and urea (110 kg N ha^-1). Broiler litter (broiler litter-treatment) was applied (4.6 Mg ha^-1). The crimson clover alone did not receive any fertilizer. Sweet potato slits were transplanted and grown for 120 days. Each year after harvest, soil samples were collected at 0-15 cm depth. The soil samples were air-dried, sieved to pass through a <2-mm mesh sieve. Total P was determined by the perchloric acid method and water-soluble P, Bray P-1, Mehlich, and Olsen-P were extracted from the samples. Results showed that application of NPK or broiler litter did not affect water-soluble P concentration in the treatments between 2002 and 2007 (54.4 mg P kg^-1 on average). However, the Bray P-1 extracted increased significantly (P < 0.05) in the NPK and broiler litter plots as compared with the control weed plot or the crimson clover plot alone.  In the NPK plots, Bray P-1 increased from ≈ 57 mg P kg^-1 in 2002 to 99.4 mg P kg^-1 in 2007. No significant (P < 0.05) build-up P was observed in the broiler litter plot between 2002 (47.8 mg P kg^-1) and 2007 (70.9 mg P kg^-1).