Geoelectric Evaluation of Subsoil for Optimum Cocoa Yield in Parts of Ondo State, Southwestern Nigeria

Abstract

Geoelectrical resistivity and direct pitting investigations of subsoils from different cocoa plantations across two senatorial districts of Ondo state, Nigeria were conducted with a view to delineating the subsoil units, identifying the nature and composition of the subsoil units and determining the influence of geology, subsoil resistivity and thickness on the growth and yield of cocoa plants. The study involved reconnaissance geological mapping, Schlumberger vertical electrical resistivity sounding and direct pitting techniques. Eight sounding data were collected with ABEM SAS 1000 resistivity meter. The interpretation of the sounding data involved partial curve matching and computer iteration techniques with direct pitting and geological data as control.  Three to four distinct subsurface geoelectric/geologic layers were identified. These included the topsoil; the weathered layer/fractured basement/cretaceous sediment and basement bedrock. The layer resistivity’s range were 126 - 2306; 37 – 1453 and ? ohm-m respectively while the thickness values of the upper two layers were 0.6 – 1.9 m; 1.9 - 25.2 m respectively. The weathered/fractured column and cretaceous sediments constituted the dominant water saturated unit.  Resistivity and thickness thresholds of 37 - 511 ohm-m and 1.9 - 19.8 m are suggestive of a significant proportion of clay and sand in the soil identified with Idanre, Oda-Akure and Ondo farm sites that usually gave optimum yields. However, other farm sites as Arimogija, Ikpemen and Ago Panu in Owo, Ibulesoro-Akure and Ile-Oluji soil profiles graded into more sandy soils with resistivity and thickness/depth thresholds of the topsoil and weathered layers between 126 - 2306 ohm-m and 5.3 – 35.2 m respectively. It was concluded that the relatively thick column of the weatherable products of the gray gneiss/charnockitic rocks as sandy clay/clayey sand of the upper two layers may have remained the most important underlying geologic units for optimal growth of cocoa in the state.

Keywords: Schlumberger, Sounding, Resistivity, Thickness, Soil, Weatherable.