GEOPHYSICAL AND PERMEABILITY ASSESSMENT OF SOIL SPATIAL VARIABILITY OF COCOA RESEARCH FARM IBADAN, SOUTHWESTERN NIGERIA

Abstract

Site specific management has been the recent trend in assessing agricultural soil quality to optimizing productivity. This research stemmed from variations observed in pods yield by cocoa trees at Cocoa Research Institute of Nigeria. Soil conductivity (EC), volumetric water content (VWC), thermal properties and soil permeability were assessed at the root zone (0.3 m) to delineate soil’s management areas within cocoa farm. Spatial evaluations were conducted during the wet and dry seasons. A total of twenty-seven lines were occupied consisting of 912 and 906 EC/VWC data points during wet and dry seasons respectively on a non-saline soil (˂2000 μS/cm) using Allied resistivity earth meter (EC) and VG-meter-200 soil moisture meter (VWC). KD2PRO was used in evaluating the thermal regime during wet (90 data points) and dry (89 data points) seasons; and permeability test conducted on ten soil samples. ECa map was generated using ARCGIS software classified the soil into zones of low, medium and high EC. Variation in VWC is synonymous to EC distribution in soils. Strong correlation of 0.971 (wet season) and 0.806 (dry season) existed between EC and VWC. Regions of high EC/VWC were characterized by soils of low thermal diffusivity, high specific heat, high thermal conductivity and weak to moderate correlations were generated between these thermal properties and EC/VWC. Soil permeability (k) studies revealed that soils from high EC section have low permeability (0.000126 to 0.000431 cm/sec) suggesting slow/moderately slow/moderate infiltration rate classified to be silty sand/dirty sand while electrically less conductive soils were characterized with high permeability (0.000437-0.004109 cm/sec) indicating moderate to moderately rapid infiltration associated with nutrient leaching and water depletion, categorized as silty/dirty sand/sand/fine sand. These techniques mapped out zones of abundant pods yield to be sections of high EC/VWC, low k and vice versa.