ORGANIC AMENDMENTS AND PHOSPHORUS DYNAMICS: I. PHOSPHORUS CHEMISTRY AND SORPTION

Abstract

The influence of the amount and kinds of organic and inorganic amendments on phosphorus (P) sorption characteristics was determined on two soils from Oregon, Jory (Xeric Haplohumult) and Tolo (Typic Vitrandept), and three soils from Rwanda, Mata (Sombrihumult), Kibeho (Paleudult), and Kinigi (Typic Placandept). Steer manure (manure), alfalfa (Medicago sativa), or wheat straw (Triticum estivum) residues were added to Jory soil samples at the rates of 1, 2.5, and 5% (wt/wt). Separate Jory soil samples were amended with CaCO₃ or CaSO₄ at 1, 1.5 or 3 times the CaCO₃ equivalent required to neutralize exchangeable aluminum (Al). Other soils received only the highest amount of each soil amendment. After 28 days of incubation, the soil samples were air-dried and P sorption isotherms, pH, exchangeable Al, and extractable phosphorus were determined. Increasing the rate of manure or alfalfa increased pH, P in equilibrium solution, and extractable P and decreased the exchangeable Al. In turn these amendments decreased values for the affinity constant, adsorption maxima, and P sorbed at 0.2 mg P L^-1. Soils amended with CaCO₃ produced results similar to manure and alfalfa but did not markedly influence the extractable P or the affinity constant. Wheat straw had a variable and negligible effect on P sorption in most of the soils. High rates of CaSO₄ decreased exchangeable Al, but did not affect the pH, extractable P, the affinity constant, and P sorbed at 0.2 mg P L^-1 (P_0.2). Overall, organic amendments with high P content, such as manure and alfalfa, may substitute for CaCO₃ as a soil amendment to increase pH and decrease P sorption capacity in soils. The magnitude of the response to amendments varied with the type of soil and amendment. However, all soils responded to the application of high rates of manure, alfalfa, or CaCO₃.