Prediction of erosion rates and the effects of topsoil thickness on soil productivity
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Part one of this study involved the determination of quantitative parameters for predicting erosion rates using runoff plots. The treatments on the runoff plots included: Bare fallow, maize with minimum tillage, maize with 3 t ha-l of maize residue, maize with conventional tillage, sole beans, and maize intercropped with beans on alternate rows. The highest runoff and soil loss was observed on the bare fallow plots followed by maize with conventional tillage, maize with minimum tillage, maize intercropped with beans on alternate rows, sale beans, and maize with 3 t ha-l of maize residue during all the seasons. The best erosivity factor for this area was found to be EI30, defined as the product of the rainstorm's Kinetic energy and its maximum 30 minute intensity, with a correlation coefficient of 0.80 and a mean annual value of -1 -1 154.92 MJ cm ha h . The mean soil erodibility factor determined using EI30 erosivity factor for the luvisol found at the study site was 0.47 t h MJ-l ern-I The mean soil erodibility factor was found to vary between -1 -1 0.11 t h MJ cm for rainstorms that produced soil -1 ~l -1 loss less than 0.5 t ha and 1~54 t h MJ cm for rainstorms that produced soil loss greater than 30 -1 t ha . - xxxvii - The mean estimates of crop cover and residue management factors C for maize with conventional tillage, maize with minimum tillage, maize intercropped with beans on alternate rows, sale beans, and maize with 3 t ha-l of maize residue were 0.77, 0.45, a~44, 0~30, and 0.15 respectively. The soil at the study site was assigned soil - ..1. -1 -1 loss tolerange value of 5.6 t ha y (0.37 mm y ) calculated using the estimated soil formation rate, the available plant rooting depth, the present soil depth, and the effect of topsoil thickness on maize production. The second part of this study aimed at the I determination of the effect of topsoil thickness on maize production and involved simulation of erosion by artificial removal of 0, 3, 6," 9, 12, and 15 cm of the topsoil and application of different rates of I fertilizer and manure. The highest maize height and maize grain and residue yields at each depth of topsoil removed were achieved on plots that had received 40 t ha-l of manure or 120 kg ha-l Nand -1 . 40 Kg ha P. The highest reduction of the maximum matze height and maize grain and residue yields per centimeter of topsoil removed was obtained on plots that had lost 12 cm of the topsoil at all fertilizer and manure rates. Maize grain and residue yields obtained from plots that had lost more than 12 cm of the topsoil were significantly lower than those - xxxviii - obtained from plots that had lost less than 9 cm of the topsoil at all fertilizer and manure rates. It was therefore concluded that loss of more than 12 cm of topsoil may result in partial or tota~ loss of this soil as a resource from maize production standpoint and any attempt to restore its productivity by application of higher fertilizer and manure rates would be uneconomical.