Changes in organic and inorganic phosphorus composition of two grassland soils and their particle size fractions during 60–90 years of cultivation

Abstract

Summary Changes are reported in the chemical and biological composition of soil phosphorus (P) in a Black Chernozemic silt loam (Blaine Lake Association) and a Dark Brown Chernozemic sandy loam (Bradwell Association) during 60–90 years of cultivation. Cultivated and adjacent uncultivated soils were sampled, separated into particle size fractions by physical dispersion and the fractions subjected to a sequential chemical extraction to remove several forms of inorganic phosphorus (Pi) and organic phosphorus (Po). In the uncultivated Bradwell soil significant amounts (7%) of secondary (NaOH extractable) Pi forms were associated with high levels of labile (bicarbonate and resin extractable) Pi. These secondary Pi forms, which were concentrated in the finer particle size fractions (<2μm), contributed to the P loss during cultivation of the coarse textured Bradwell soil, whereas all P loss in the Blaine Lake soils was due to Po losses alone. Sulphuric acid extractable P (thought to be mainly apatites) accumulated in both soils under cultivation, particularly in the coarse silt (50–5 μm) fraction. Labile P fractions were greatly reduced during cultivation, indicating a significant reduction in available P and P fertility of cultivated soils. This reduction in P fertility was closely tied to soil organic matter losses.