General and Spatial Variability of Soil Under Vetiver Grass Strips

Abstract

ABSTRACT The variability of surface soils on the same field is influenced largely by soil erosion and runoff deposition. The general and spatial variability of soils on runoff plots under the influence of vetiver grass strips (VGS) was therefore studied using both the conventional “Fisher” statistics and geostatistics. The focus was on safe use of vetiver system for combating land degradation in view of its various advantages. The overall objective was to quantify the variability in relation to VGS effects on the soil and its influence on other plants. The VGS were established at 20 m spacing two years earlier, hence some soil had accumulated on the upper slope side at a mean rate of 71.0 mm per year. There were greater faunal activities around the VGS than elsewhere in the field. The study area was on a 5% slope and grid (5 m × 3 m) systematic sampling was used; the samples were collected at depths of 0-5 cm and 5-10 cm were processed and analyzed physically and chemically. Static soil properties such as total sand (ranged 712 to 866 g kg−1) and bulk density (1.24 to 1.51 g cm−3) had low co-efficient of variation (CV) values (CV < 15%), and dynamic soil properties, which could easily be influenced by management, (such as available phosphorus, P, and silt; ranging from 2.85 to 13.2 mg kg−1 and 64 to174 g kg−1, respectively) had high CV values of 35% to 70%. Gravel content (50.4-175.1 g kg−1) was not significantly different amongst plots in the field. All the parameters analyzed followed a normal frequency distribution. For the geostatistics, considered along a south-north transect only, the semivariances approached the overall variance, Ko, within distance lags of 5-20 m for clay at 5-10 cm depth, total sand and soil pH at 0-5 cm depth. Values beyond the variance distances, Ko, were considered independent of each other. Bulk density, available P, and soil pH revealed no spatial dependence on the VGS effect. The results of the effects of VGS on properties of soils bound by it showed that clay and bulk density were less with VGS than without; 83.83 g kg−1 clay with VGS and 90.5 g kg−1 without VGS; bulk density of 1.40 g cm−3 with VGS and 1.43 g cm−3 without VGS both at 0-5 cm depth. The reverse was observed for sand content, though; their effects were linked to slope, erosion and pedogenic factors rather than VGS. Among the chemical properties, organic carbon (OC) and total nitrogen (TN) were higher in parcels of land bounded by the VGS than those without. At 5-10 cm depth, OC was 14.25 g kg−1 with VGS and 11.35 g kg−1 without. Similar rise was observed for soil pH; 5.61 with VGS and 5.25 in the control without VGS at 0-5 cm depth. In contrast, available P was lower in lags bounded by VGS than those without VGS. The use of the VGS technology in combination with mechanical measures such as along water ways, gully embankments, roadsides and sloppy farm-lands to protect and rehabilitate marginal lands to facilitate crop production, and the integration of soil variability with problem solving were recommended. It was concluded that since VGS did not cause any significant difference in the soil physical properties it can be safely used as an environmentally-friendly natural resource to improve and protect farmlands and infrastructures. However, the application of geostatistics was not necessary because of the small volume of the data involved, conventional statistics were enough to fully quantify the variability.