ORGANIC AMENDMENTS AND PHOSPHORUS DYNAMICS .2. DISTRIBUTION OF SOIL-PHOSPHORUS FRACTIONS

Previously, we found that organic amendments decreased P sorption that was related to changes in some soil chemical properties (e.g., pH and exchangeable Al). However, addition of organic residue to soils may a lso affect P sorption by adding PO4 (P-i) or releasing organic P (P-o) during mineralization of residues and affect redistribution of P amon g P pools in soils. The objective of this study was to determine the e ffects oforganic amendments (manure, alfalfa (Medicago sativa) and whe at straw (Triticum estivum) compared with inorganic amendments (CaCO3 and CaSO4) on P fractions, after 28-day incubations. The five acid soi ls with high P fixation characteristics used in the study were Jory (c layey, mixed, mesic xeric Haplohumult) and Tolo (medial over loamy, mi xed, frigid typic Vitrandept) from Oregon, and Mata (Sombrihumult), Ki nigi (Placandept), and Kiheho (Paleudult) from Rwanda. Manure and alfa lfa residues increased resin P-i and NaHO3-P-i (biologically available ) and NaHCO3-P-o (readily mineralizable) and NaOH-P-i (chemisorbed) in all soils. The Tolo soil showed the least increase of chemisorbed P-i and the greatest increase of resin and NaHCO3-P-i, which corresponded to previous findings of lower P sorption by Tolo than by the other so ils. Wheat straw residues had relatively little effect on P fractions as a consequence of its low total P content In all soils, both NaOH-P- i (and to lesser extent NaOH-P-o) increased with manure or alfalfa ame ndments. These fractions are sites for P sorption; thus, increases in these fractions provide evidence that P-rich organic amendments can de crease future P sorption in soils by reacting with these sorption site s. Calcium carbonate or CaSO4 . 2H(2)O had little or no effect on P di stribution, indicating that redistribution of P fraction does not occu r with reduction in exchangeable Al and Fe (CaSO4 and CaCO3) or change in pH (CaCO3).