ABSTRACT

Knowledge of the soil minerals as they relate to phosphorus availability is an important index in understanding phosphorus behaviour in soils. Detailed study of the clay minerals as they affect the phosphorus forms, adsorption and kinetics in some soils of contrasting parent materials viz: Coastal Plain Sand, Sandstone, Basalt and Shale at different soil horizons (0-20cm, 20-40cm and 40-60cm) in South Eastern Nigeria was carried out. Soil samples were collected from the different parent material in three replicates and a total of thirty six composite samples were collected. The experimental design was a 4 x 3 factorial in Randomized Complete Block Design (RCBD). Statistical analysis revealed significant (P<0.05) variation in soil physical and chemical properties with parent material and soil depth except OM, TN, Mg 2TEA and BS where variations were not significant. However, interaction between parent material and soil depth was only significant in influencing the pH. Top soil layers had significant greater amount of OM, TN, AP. Cations, Sand and Silt. Generally, soil of shale formation was superior, followed by basaltic soil in these soil fertility indicators, while soil of Coastal Plain Sand formation had the least amounts. The mineralogy of the clay-sized fraction of the soils showed the dominance of kaolinite and quartz with some amounts of Smectites and Montmorillomte at lower soil horizons especially soils formed on Shale. Hematite and goethite were the predominant pedogenic iron oxides and gibbsite which is an aluminum oxide was also identified. While the concentrations of quartz and kaolinite decreased with depth, the occurrence of smectites in mostly soil of shale formation increased with soil depth. Significant variation (P><0.05) on the amount of the different P forms with respect to parent material and soil depth were observed. However the interactive effects of parent material and depth on the P forms were not statistically significant. The mean values of total p in relation to parent materials was in the order of Shale (463.00mg/kg)> Basalt (338.00mg/kg)> Coastal Plain Sand (245.00mg/kg)> Sandstone (313.00mg/kg). Organic P values ranged from 75mg/kg in soil of Basalt formation to 91.6mg/kg in soil formed on shale. The relative abundance of the various forms of inorganic P fractions was in the order: inactive P> Fe-P> Al-P >Ca-P. Results also showed non-significant variations of SPR with parent material and soil depth. The SPR mean values ranged from 64.1mg/kg in Basaltic soil to 24.6mg/kg in soil Coastal Plain Sand formation and the SPR, irrespective of parent material increased with soil depth. Freundlich model better described P adsorption properties of soils of Coastal Plain Sand and Sandstone formations than Langmuir model and the Langmuir equation described the P sorption data from soils of basalt and shale formations better than the Fruendlich equation. Based on the Freundlich equation, the P adsorption capacity (a) ranged from 151.0mg/kg in Basaltic soil to 86.9mg/kg in soil of Coastal Plain Sand formation and the "a" values of the soil were in decreasing order of Basalt> Shale> Sandstone> Coastal Plain Sand derived soils. Both -4 parent material and soil depth as lone factors significantly influenced the Ki, but their interactive effect was not significant. The means Ki values occurred in the following order: Shale (5.05m')> Basalt (4.67 m 1 )> Sandstone (3.55 m1 )> Coastal Plain Sand (1.08 m'). The rate constant (Ki) generally increased with soil depth. Relating the rate constant (Ki) to clay mineralogy showed that the values of Ki were highest in soils derived from Shale and Basalt which had the highest concentration of Fe and Al oxides. In view of the above, an average of 40.50mg/kg of P should be applied to meet the SPR of the soils. Freundlich and Langmuir equations are recommended for description of sorption data, while First Order Reaction model is suitable for predicting the rate of soil P adsorption in soils of Southeastern Nigeria.