Soil degradative effects of slope length on alfisols in western Nigeria. IV plots of equal land area

Abstract

Field experiments were conducted to evaluate effects of four slope lengths (20 m, 10 m, 6·7 m and 5 m) and four slope gradients (0·9–1·8 per cent, 4·7–5·7 per cent, 9·3–9·9 per cent, and 14·2–14·9 per cent) on interrill erosion. Plot width was appropriately adjusted to keep the plot area constant at 20 m2. These experiments were conducted on natural slopes and rainfall conditions for four consecutive years from 1984–1987 for maize (Zea mays)–cowpeas (Vigna unguiculata) or soybean (Glycine max)–cowpeas rotation. Runoff per unit area decreased with increase in slope length especially for steep slope gradients and erosive rains. Mean annual runoff per unit area was 3·2 per cent, 3·7 per cent, 4·6 per cent, and 8·1 per cent of the annual rainfall amount for 20 m, 10 m, 6·7 m and 5 m slope length, respectively. Similarly, interrill erosion per unit area also decreased with increase in slope length, and was 893 kg ha−1, 588 kg ha−1, 845 kg ha−1 and 1979 kg ha−1 for 20 m, 10 m, 6·7 m and 5 m slope length, respectively. For the steep slope gradient of 14·2–14·9 per cent, mean sediment concentration was 0·15 g l−1 for 20 m, 0·19 g l−1 from 10 m, 0·56 g l−1 for 6·7 m and 0·92 g l−1 for 5 m slope length. Highest grain yield was obtained for slope gradient of 4·7–5·7 per cent for maize, and for 0·9–1·8 per cent slope for cowpeas. High soybean yield was obtained on the steep slope gradient. Crop yields were related to clay content of the surface horizon and the soil's susceptibility to crusting, but not to slope length. These results show that surface detention capacity, for runoff and sediments, play an important role in runoff rate and amount and on interrill erosion. © 1997 John Wiley & Sons, Ltd.