A comparison of different Grasses as filter strips for Soil and Water conservation on Crop Land
The experiment involved comparing three types of grasses when used as filter strips for soil and water conservation on cropland. The slope of the land was approximately 8%. The soils were the eutric nitosols developed from the Nairobi trachytes (Ga chene , 1989). These soils are deep and well drained. The three grasses tested were the napier grass (Pennisetum purpureum), Nandi setaria (Setaria anceps) and the tall signal grass (Brachiaria ruz t z Iens Ls ). The experimental design used was the completely randomised block design (CRBD). This comprised of three blocks that were 3 m apart. Each block comprised of four plots that were 2 m wide, 11.6 m long and 0.5 m apart. The four treatments (the control and the three types of filter strips) were randomly distributed within each block using random number tables. The grass filter strips (0.5 m wide) were established at the lower end of the plots using splits from the nearby grass bulking site. Pure stand crops of maize and beans were planted during the long and short rains respectively. xix The run-off and eroded sediments call ection and storage involved a collector unit at the downstream side of each pi at. From the call ector, the run-off and the sediments were conveyed to the main run-off storage tank (approximately 1 m3) through a pvc pipe. The pipe was 3 m long and 6.4 cm in diameter. A small drum of approximately 0.09 m3 was put inside the main run-off storage tank. The small drum was for storing run-off from I ight storms an d also most of the eroded sediments were stored here for ease of removal and measurement. Apart from run-off and soi I loss, other parameters measured were the crop height, crop yields, so i I moisture and the fodder potential of the grasses. The four treatments showed no significant difference in run-off and soil loss reduction during the long rains of 1990. This was well shown by the low soil loss reduction efficiencies. The efficiency during this season was 2.6, 4.0 and 13.6% for the napier, setaria and brachiaria respectively. This improved during the short rains of the same year whereby, the napier filter strip attained 17.7%. The setaria only improved to 5.9% while the brachiaria had 58.8%. During the long rains of 1991, the napier strip gave a run-off reduction efficiency of 40.5%, the setaria filter strip had 29.7% xx while the brachiaria one had 68.9%. Similarly, the treatments had no significant difference in soil loss reduction during the long rains of 1990 but improved with time as in the case of run-off reduction. The napier, setaria and brachiaria strips had a soil reduction efficiency of 3.8%, 7.9% and 19.0% respectively during the long rains of 1990. For the short rains of the same year, the napier strip had a soil reduction efficiency of 93.0%, the setaria strip had 81.9% while the brachiaria one attained 94.7%. The strips maintained high efficiency in soil loss reduction during the long rains of 1991 whereby, the napier filter strip had an efficiency of 75.6%. The setaria and brachiaria strips had 67.5% and 92.8% respectively. In overall run-off reduction efficiency, the filter strip of brachiaria was the most effective followed by the napier and lastly the setaria filter strip. The filter strips were noted to have minimal adverse ef fects on the nearby crop. I t was on I y the napi er which was observed to a substantial adverse effect on the first one or two rows of the adjacent crop. Where a filter strip was planted, one row of maize or two x xi rows of beans were foregone. In overall crop yields, the treatments had no significant difference. Though the filter strips were found to utilize the soil moisture around them, they conserved more a short distance away from the strip edge. In the case of fodder potential, the strips of napier, setaria and brachiaria had an annual dry matter yield of 8.8, 2.5 and 2.7 Kg/m2 respectively. This was from green fodder- y i e ld s of 46.7, 12.4 and 11.7 Kg/m2 for the napier, setaria and brachiaria respectively. Together with important nutrients content, the napier was the most superior fol lowed by the brachiaria. The benefits of the fodder and the conservation of the soil and water were found to compensate for the crop area taken up by the grass filter strips.