Green Structure for Soil and Water Conservation on Cultivated Steep Land

Abstract

A study was conducted on a farmer’s field on a 35 % slope in the humid forest zone, Nigeria on the use of a green structure that performed some engineering functions on cultivated steep lands to help curtail soil and water losses. Treatments consisted of a vetiver grass strip as a green buffer structure at different surface spacings of 5, 15, 25 m with no vetiver as the control and were laid out in a randomized complete block design with three blocks. Twelve erosion plots each measuring 50 m long and 3 m wide were used for the study. The plots were planted with cassava and maize in a simple crop mixture. The runoff, soil loss and crop yield were assessed under the four treatments. The rainfall lost as runoff with no vetiver (control plot) was 24.8 % compared to 7.7, 11.5 and 11.6% lost on the green structure plots at spacings of 5, 15 and 25 m, respectively. Soil loss on the no vetiver plot was 40 times higher than the acceptable soil loss limit of 12 t.ha-1 yr-1 for the tropics, whereas the soil loss on plots under vetiver at spacings of 5, 15 and 25 m was 1.4, 6.8 and 6.5 times higher than the acceptable limit. The maize grain yield and the fresh cassava tuber yield were significantly lower in the control plots and highest with the 5 m spacing. A vetiver strip as a green structure was more effective at a spacing of 5 m as it reduced soil and water losses and increased crop yield. This spacing was also adequate for traditional pre- and post-farming activities.