Soil degradative effects of slope length and tillage method on alfisols in Western Nigeria II. Soil chemical properties, plant nutrient loss and water quality

Abstract

Effects of erosion-induced degradation on soil chemical quality, plant nutrient loss in runoff water and water quality were related to slope length for a 5-year study from 1984 through 1988 conducted on tropical Alfisols under maize (Zea mays)–cowpeas (Vigna unguiculata) rotation. A total of 13 field runoff plots involved six slope lengths (i.e. 60 m, 50 m, 40 m, 30 m, 20 m, and 10 m), two tillage methods (i.e. conventional till and no-till), and an uncropped control of 25 m length. Slope length effects on soil chemical quality differed among tillage methods. Decrease in slope length had a favorable effect on soil chemical quality in the conventional till treatment for exchangeable Ca+, cation exchange capacity (CEC) and Bray-P. The soil organic carbon (SOC) increased with decrease in slope length at the rate of 0·007 per cent m−1 for 0–5 cm depth and 0·015 per cent m−1 for 5–10 cm depth, and soil pH decreased with increase in slope length. Soil concentration of Bray-1 P generally increased with decrease in slope length. For the no-till treatment, however, there were no consistent trends in soil chemical quality with regard to slope length. Further, soil chemical quality was superior under no-till than conventional till treatment for 0–5 cm and 5–10 cm depths. All soil chemical properties declined with cultivation duration, with the rate of decline being more in conventional till compared with the no-till treatment. The rate of decline for the conventional till treatment was 0·396 per cent year−1 for SOC, 0·21 unit year−1 for pH, 0·053 per cent year−1 for total soil N, 0·375 cmol kg−1 year−1 for Ca+, and 0·41 cmol kg−1 year−1 for CEC. The rate of decline in soil quality with cultivation duration was consistently more for longer than for shorter slope lengths especially for SOC and soil pH. Temporal changes in soil chemical quality for the no-till treatment were not well defined. The total loss of plant nutrients in runoff ranged from 8·4 kg ha−1 to 17·8 kg ha−1 for season 1, and, in general, the nutrient loss decreased with decrease in slope length. The total loss of plant nutrients in season 2 was lower than that in season 1, and lower with no-till compared with conventional till treatment. Total nutrient loss in runoff decreased with increase in slope length for conventional tillage, and increased with decrease in slope length for the no-till treatment. Concentration of plant nutrient in runoff differed among elements, tillage methods and slope length treatments. Concentration of bases and NO3-N in runoff followed a U-shape response over time during the year, with relatively high concentrations toward the beginning and end of the rains. The concentration of PO4-P in runoff generally decreased with time after application of fertilizer. For the conventional till treatment, the concentration of PO4-P, K, and Fe decreased with decrease in slope length. © 1997 John Wiley & Sons, Ltd.