CHANGES WITH TIME IN FORM AND AVAILABILITY OF RESIDUAL FERTILIZER PHOSPHORUS IN A CATENARY SEQUENCE OF CHERNOZEMIC SOILS

Abstract

Granular monoammonium phosphate (500 μg P/g soil) was applied in the field to three soils of an Oxbow catena. Changes in P form during the ensuing 2½ yr were determined by total P and modified inorganic P fractionation analysis; an index of available P was obtained by NaHCO3 extraction. The original soils were very low in NH4Cl-soluble P and available P; inorganic P apparently existed as sparingly soluble minerals. After application, most of the fertilizer P remained in the Ap horizon. In the Calcareous and Orthic profiles after 2 mo, 52 and 67%, respectively, of applied P recovered as inorganic P remained in available forms, mainly as NH4Cl-P. Subsequently in the Calcareous profile, NH4Cl-P underwent conversion to NH4F-soluble forms, causing a further 70% reduction in P availability. In the Orthic profile, the only apparent change with time was a decline to 41% in P availability. Applied P transformation to H2SO4-P was minimal in both soils. In the Gleysol profile, only 31% of P in the inorganic reaction products, which consisted mainly of NH4F-P and NaOH-NaCl-P, remained as available P after 2 mo. However, no further reduction in P availability occurred with time. Results indicated that an appreciable portion of residual fertilizer P in Chernozemic soils may persist for years in readily available forms. Soil pH strongly influenced residual P form and availability.