Journal of Agricultural Science and Practice

Predictive accuracy of a soil map is the scientific requirement for its use. To determine the extra information contained in a manuscript soil map (1:250,000) of Imo State, in the coastal plains sand zone of the state, its predictive accuracy was compared with that of the published (1:1,000,000) version. The study was done with 106 soil samples randomly taken in the location of study with the auger at two depths, 0-20cm and 20-40cm, which were analysed using standard laboratory procedures to determine physic-chemical properties – sand, silt, clay, silt/clay, pH, Exch. Ca, Mg, K, Na, Al, H, Al+H, ECEC, BS, ESP, OC, TN.. Data were statistically analysed using Coefficient of Variation (CV%), Analysis of Variance (ANOVA) with means separated at a 5% level of significance, and the technique based on relative variance (I-RV). Both maps had better predictive accuracy for alluvial soils than well-drained units delineated based on minor relief differences. Despite high within unit variation of properties, map units generally differed significantly at 5% in terms of sand, silt, silt/clay, Al, H, Al+H, ECEC, BS, ESP, OC, K. Predictive accuracies (1-RV) for the parameters ranged from 0.01 for pH to 0.62 for BS. Sand and silt had relatively good predictions at both depths and scales, with values ranging from 0.42 to 0.57. The parameters silt/clay, K, Al, H, ECEC, and Al+H, had better prediction at subsoil with values ranging from 0.26 to 0.58. The manuscript map had improved prediction at the topsoil for Al, H, Al+H, BS, ESP, OC, and TN with values improving from a range of 0.04 – 0.25 to one of 0.15 – 0.62. At the subsoil, the same map had improved prediction for silt/clay ratio with values changing from 0.17 to 0.32. Despite being at a small scale too, the manuscript soil map brought out the soil’s potential better, which fact makes it a better base map for more detailed mapping of the soils than the published one. The study thus recommends making reference to the manuscript map for further studies.