ABSTRACT
This study was carried out in Akoko Edo LGA, Edo State, to compare the use of Geographic Information System (GIS) and conventional soil survey methods in the identification, mapping and assessment of the soils of the Local Government Area. Two methodologies were employed: GIS assisted soil survey methodology was used to delineate the entire area into different mapping units considering the active soil forming factors in the area to form thematic layers which eventually were overlaid on each other to produce the soil map with five representative pedons which were sunk, described, sampled and analyzed. Mapping of the soils of the area using the semi detailed soil survey methodology was employed. Eleven mapping units were delineated from the area with auger borings at 1000 m by 500 m apart which gave 2,592 auger examination points. Eleven pedons were sunk, described and sampled. Soil samples were analyzed.
Soils were classified according to USDA and WRB systems. Suitability evaluation for Oil palm, Cashew and Cocoa was done by using established guidelines. Coefficient of Variation (CV), Intra-class correlation coefficient (Pi), Relative Variance (RV) and Purity percentage were used for variability assessment of the methods and maps.
GIS procedure showed that pedons 1 to 5 were largely Inceptisols/Cambisols while with the conventional procedure, the soils, with eleven mapping units were classified as Inceptisols/Cambisols and Alfisols/Lixisols. The suitability for the selected crops revealed that for GIS procedure, 67,750 ha (52.2%) was suited for oil palm cultivation at moderate level and 61,896.8 ha (47.8 %) marginally suited for oil palm production. 76931.5 ha (59.3 %) moderately suitable for the cultivation of cashew and 52,715.3 ha (40.7 %) marginally suited for cashew. Cocoa was found to be marginally suitable for the whole research area with different limitations in each mapping unit. However, the conventional procedure revealed that for oil palm, 58,007.8 ha (44.8 %) was moderately suitable, 42,204.2 ha (32.5 %) marginally suitable, and 7,472.3 (5.8 %) currently not suitable with limitations in topography and soil wetness parameters. Cashew was found to be moderately suitable for an area of 69,060.5 ha (53.3%) and marginally suitable (S3) for an area covering 38,623.8 (29.8 %). 122,174.5 ha (94.2 %) was found to be marginally suitable (S3) for cacao cultivation and 7,472.3 ha (5.8 %) currently not suitable (N1) for cacao production with limitation in topography. The indices of comparison for the two methods as shown in the assessment of the purity, reliability and predictive capacities of the two maps procedure revealed that the conventional procedure is more reliable and superior to the GIS procedure. Considering the time and cost of using both procedures, GIS procedure is faster and less capital intensive.
