Spacing effects of vetiver grass (Vetiveria nigritana Stapf) hedgerows on soil accumulation and yields of maize–cassava intercropping system in Southwest Nigeria

Large losses of soil usually occur under continuous cropping systems in the tropics owing to high rainfall erosivity coupled with predominately weak soil structure. Vetiver grass (V. nigritiana) planted at intervals across slopes could check erosion losses and allow meaningful cultivation of the alleys between vetiver hedgerows. Between 2004 and 2007, studies were conducted at Ibadan in the humid region of Southwest Nigeria to evaluate the effects of vetiver grass hedgerows (VGH) on soil accumulation, bulk density, soil moisture content, infiltration characteristics, crop growth and yields. Plots were planted to cassava intercropped with maize. Treatments consist of a control (no hedgerow) and three VGH at 5 m (8 hedges), 10 m (4 hedges) and 20 m (2 hedges) spacing on 13% slope of farmland. Treatments were laid out in a randomized complete block design with three replications. Measurements of soil physical properties were taken at the center of the control plot and 0.25 m in front of vetiver hedgerows in plots with vetiver. Calibrated metal rods were inserted 0.15 m from the edge of the hedgerows to monitor accumulated soil due to runoff while erosion pins were inserted every 5 m down the slope on control plots to measure the depth of soil removal. ccumulation by VGH serving as barriers to trap eroded soil was significantly (p < 0.05) affected by the spacing. The average depth of soil accumulated per VGH spaced at 5 m, 10 m and 20 m wide intervals over three years were 32.17, 67.21 and 159.7 mm respectively while depth of soil removed on the control plot was 168.0 mm. Gravimetric moisture content increased with wider alley spacing and infiltration characteristics were improved in the alleys. Mean maize grain yield over the three growing seasons was significantly (p < 0.05) higher in vetiver grass alleys of 5, 10 and 20 m than the control by 34.4%, 23.3% and 6.5%, respectively. Cassava tuber yields were higher in vetiver alleys than the control by 1.7 to 4.7% in the first season, 2.8 to 5.1% in the second season and 3.6 to 7.0% in the third season, following the same trend as maize yields. The greater loss of soil under maize and cassava intercropped on plot without vetiver grass resulted in a decrease in the yields of crops as compared to vetiver alley plots. Although crop yields in VGH plots were higher than on the control, areas of land taken out of cultivation by VGH spaced at 5, 10 and 20 m wide intervals were 120, 60 and 30 m2 ha− 1, respectively at the end of the third season when the vetiver was 52 months old. Thus, a balance needs to be struck between the number of hedges with respect to slope and the area of land left for cultivation.