EVALUATING THE EFFECTIVENESS OF FOUR SELECTED METHODS OF INVESTIGATING SOIL EROSION EFFECT ON SOIL PRODUCTIVITY IN NSUKKA, ENUGU STATE, NIGERIA
This study was conducted in 2012 at the University of Nigeria, Nsukka Teaching and Research Farm (UNN), and Ekwegbe, both in Nsukka agricultural zone; to evaluate the effectiveness of four selected methods of quantifying erosion effect on soil productivity at Nsukka, southeastern Nigeria. The four methods were (1) desurfacing technique (DT), (2) Neill’s (1983) productivity index, (3) modified productivity index (MPI) and (4) Riquier’s productivity index (RI). Soils were sampled at 0-30, 30-60, and 60-90 cm depth zones at each location prior to planting and after harvest. Incremental depths (0, 2, and 4 cm) of topsoil layers were manually removed to simulate erosion at the two sites. Poultry manure (10 t ha-1) was applied two weeks before planting as a soil amendment. Correlation and regression analyses revealed that RI was significantly (p < 0.05) correlated (positively) with plant height at 10 WAP (r = 0.75*), LAI at 14 WAP (r = 0.76*) and pod yield (r = 0.72*) at UNN, and was ranked first in effectiveness, followed by DT, which had a significant (p < 0.05) negative correlation with plant height at 6 WAP (r = -0.45*), while PI and MPI were less effective. At Ekwegbe, DT showed significant (p < 0.05; p < 0.01) negative correlations with plant height at 10 (r = -0.42*) and 14 WAP (r = -0.66**), and LAI at 14 WAP (r = -0.52**), and was validated as the most effective index, whereas RI, PI and MPI were less useful. Based on RI, the soils at UNN and Ekwegbe had productivity index ratings of 15% and 8%, respectively, placing them in the productivity class IV (poor productivity). Following the application of poultry manure and tillage, the soils recorded potentiality index ratings of 22% and 13%, respectively, raising the potentiality crlass of the UNN sandy clay loam soil to III (average potentiality), while the Ekwegbe sandy loam soil remained in class IV (poor potentiality). The computed coefficients of improvement (Ci) were respectively 1.5 and 1.6 for the soils at UNN and Ekwegbe. The findings indicated that although the current productivity levels of the soils were poor, there was room for improvement, given necessary management practices.