ABSTRACT

This study was carried out in Imo State, Southeastern Nigerian to investigate the effect of clay minerals from selected parent materials (Coastal plain sand, Alluvium, Imo Clay Shale, False Bedded Sandstone and Upper Coal Measure) at different soil depths (0-20, 20 - 40 and 40 – 60cm) on soil potassium dynamics including forms, adsorption properties, and kinetics. Soil sampling was carried out with soil auger in three replicates from each parent material giving a total of forty-five composite samples. The experimental design was a 5 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 Magnesium, Available Phosphorus, Total Exchangeable Acidity and Effective Cation Exchange Capacity that were not significant. However, interactions between parent materials and soil depth were not significant in influencing all the physical and chemical properties of the soils studied. With the exception of available phosphorus, soil of Imo clay formation was most superior in soil fertility variables, while the least values of these properties were found in soil of coastal plain sand formation. With respect to depth, top layers had significant greater amount of these fertility indices. The mineralogy of the clay-sized fraction of the soils showed the dominance of kaolinite and quartz with some amounts of Smectites and Montmorillonite at lower soil horizons especially soils formed on Imo clay Shale and upper coal measure. Haematite 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, goethite and hematite in mostly soils of Imo clay Shale and upper coal measure formations increased with soil depth. The potassium forms varied significantly (P<0.05) with parent material, soil depth and their interactions. Irrespective of parent material and depth, solution potassium (mean of 0.067 cmol/kg) had the least values, while the highest amounts of K were observed in the structural K fraction (mean of 11.786 cmol/kg). Most of the minerals except quartz correlated negatively with solution K and positively with structural K. Freundlich and Langmuir models sufficiently described potassium adsorption properties of soils. Irrespective of the adsorption model used, the K adsorption capacity was in the increasing order of Coastal Plain Sand > alluvium > False Bedded Sandstone > upper coal measure > Imo clay shale. Apart from quartz, other minerals especially hematite, goethite, gibbsite, kaolinite and smectite related positively with K sorption capacity and energy coefficient. Both parent material and soil depth as lone factors significantly influenced the kinetic rate constant (Ki), but their interactive effects were not significant. The means Ki values occurred in the following order: Coastal Plain Sand (21 x 10^-3 m^-1) < alluvium (22 10^-3 m^-1) < False Bedded Sandstone (38 10^-3 m^-1) < upper coal measure (38 10^-3 m^-1) < and Imo clay shale (49 10^-3 m^-1). The rate constant (Ki) generally increased with soil depth. In the same vein, goethite, haematite, smectite and kaolinite correlated positively with Ki, while the correlation between Ki and quartz was negative. In view of the above, K nutrition  may be a constraint to crop productivity in these soils due to high K adsorption capacity and adsorption rate, unless there is high K fertilization and reduction of adsorption through liming and organic matter addition. Freundlich and Langmuir equations are recommended for description of sorption data, while First Order Reaction model is suitable for predicting the rate of soil K adsorption in soils of Imo State, Southeastern Nigeria.