PHOSPHORUS SORPTION ISOTHERMS AND PARAMETER ESTIMATES OF SOILS OF THE SAVANNA, AND EFFECT OF SOIL PROPERTIES ON THESE PARAMETERS

The phenomenon of phosphorus adsorption in soils is widespread because of its agricultural importance. Crop yields are often low due to low soil solution P concentration in high P fixing soils. In view of this fact, a study was initiated to determine the P sorption characteristics and derive parameter estimates relevant in the prediction and management of P in major soils of the Nigerian savanna. To achieve these objectives, soils were sampled from 23 locations in 7 mapping units across the Nigerian savanna ecological zones. The soils were characterized and the phosphate sorption isotherms and parameter estimates were determined. The adsorption isotherms for the soils were gentle to steep, covering a wide range of P sorption characteristics. The Langmuir plots gave gentle curves in most cases, indicating that the assumptions on which the model was based were not fully satisfied. The Langmuir parameter estimates varied widely for the soils. The energy coefficient (K) ranged from 0.015 - 11.593ml/|i-1 and was particularly high for soils of the Jos Plateau with appreciable amounts of Fe oxides. The standard phosphate requirement (SRP) ranged from 2.87 - 1021 ^g/g-1, which implies that P amendment (fertilization) would be necessary for good crop growth in a good number of the soils. The effects of soil properties on P sorption parameters were evaluated. The single factor that influenced P sorption parameters appreciably was Dithionite extractable Fe (Fed). Generally, Fed, pH and CEC explained over 50% of the variation in the sorption parameters in these savanna soils.