SOIL PHOSPHORUS FRACTIONS AS INFLUENCED BY CONCRETIONARY NODULES IN NON-RESPONSIVE SOIL IN THE NIGERIAN SAVANNA.

Abdu, N* and Emmanuel, D.

The soils of Nigerian savanna are enriched with iron (Fe) concretions that have very high affinity for phosphate (P). These soils are extremely poor in P and little is known of the dynamics of P in these Fe-enriched soils. Soil samples were collected from cultivated farmers land in Biye, northern Nigeria and separated into soil fines, soil + concretion and concretions only. Phosphorus fractions (water soluble-P (H2O-P), bioavailable-P (NaHCO3-P), potentially bioavailable-P (NaOH-P), acid soluble-P (Ca-bound P/HCl-P), and residual-P) were determined in soil fines, a mixture of soil and concretions and in concretions alone following the Hedley et al fractionation scheme. The concretion contains 2 - 3 times total P more than the associated soil fines ranging, from 604 – 1014 mg/kg. Likewise, the concentration of plant available P was on the average 2 – 3 times higher in soils without concretions or fine soils than in the associated concretions. Similarly, bioavailable P extracted by NaHCO3 in soil fines (8 – 10.9 mg/kg) is 2 - 3 times greater than available P extracted from concretions. The observed P distribution was thus attributed to preponderance of Fe and manganese (Mn) oxides whose concentrations were twice as high in concretions than in the fine soils. Incorporation of plant residues and other organic materials to increase soil organic P for long- term soil fertility improvement and maintenance is highly recommended in this soil.

Key words: Concretions, Fe-nodules, P-dynamics, P-fixation.