Kinetics of Phosphate Desorption from Different soil test P Status Soils as Influenced by Incorporation of Organic Amendments and its Uptake by Flooded Rice

Phosphorus (P) build-up in soil after excess P applications can serve as a P source for succeeding crops. This study investigated the use of various organic materials, including rice-residue biochar, farmyard manure (FYM), poultry manure (PM), green manure (GM), and wheat straw (WS) for their ability to release accumulated P in soils with different soil test P status levels. Bulk soil samples (0-0.15 m) of medium-P (12.5-22.5 kg P ha(-1)) and high-P (22.5-50.0 kg P ha(-1)) status soils were collected from an ongoing long-term field experiment having different soil test P build-up levels. Soil samples were used for incubation study under flooded environments over 30 and 60 days. The kinetics of P release in soils was determined by successive extractions of P with 0.01 M CaCl2 after repeated 4 h intervals at 25 +/- 1 degrees C. The findings indicated that incorporating organic wastes increased cumulative P release in both medium-P and high-P soils in the order of FYM>PM>biochar>GM>WS. P release was found higher in high-P soils than in medium-P soils at 30 days than at 60 days of incubation. The amount of P desorbed decreased gradually as the number of extractions increased demonstrating P desorption as a slow process. The Parabolic diffusion model was found to be best for estimating P release.Among the organic materials, FYM promoted P release from soils with different soil test P levels, with higher release in high-P soils, following a slow desorption process best described by the Parabolic diffusion model.