ORGANIC AMENDMENTS AND PHOSPHORUS DYNAMICS .1. PHOSPHORUS CHEMISTRY AND SORPTION

The influence of the amount and kinds of organic and inorganic amendme nts on phosphorus (P) sorption characteristics was determined on two s oils from Oregon, Jory (Xeric Haplohumult) and Tolo (Typic Vitrandept) , and three soils from Rwanda, Mata (Sombrihurnult), Kibeho (Paleudult ), and Kinigi (Typic Placandept), Steer manure (manure), alfalfa (Medi cago sativa), or wheat straw (Trititcum estivum) residues were added t o Jory soil samples at the rates of 1, 2.5, and 5% (wt/wt), Separate J ory 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 s orption isotherms, pH, exchangeable Al, and extractable phosphorus wer e determined. Increasing She rate: of manure or alfalfa increased pH, P in equilibrium solution, and extractable P and decreased the exchang eable Al. In turn these amendments decreased values for the affinity c onstant, adsorption maxima, and P sorbed at 0.2 mg P L(-1). Soils amen ded with CaCO3 produced results similar to manure and alfalfa but did not markedly influence the extractable P or the affinity constant. Whe at 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 amendm ent to increase pH and decrease P sorption capacity in soils. The magn itude of the response to amendments varied with the type of soil and a mendment. However, all soils responded to the application of high rate s of manure, alfalfa, or CaCO3.