A two –year (2004/2005) field investigation was carried out on the runoff plots at the University of Nigeria Nsukka farm, to monitor the effects of cover management practices on physical properties, runoff and soil loss in Nkpologu sandy loam soil. The management practices were barefallow (BF), cocoyam (CY) sorghum (SG), legume (CP) and grass (PM), under no-till practice. There was no change in soil texture due to treatments. The treatments generally increased soil organic matter content compared with the control. Bulk density was significantly increased in all treatments with highest value (1.65Mg/m3) in barefallow and lowest value (1.49 Mg/m3) in grass. There was no significant decrease in porosity and pore size distribution. Mean weight diameter (MWD) of aggregates and saturated hydraulic conductivity (Ksat) were significantly increased (p = 0.05). The least values for MWD (1.06mm) and for Ksat (25.80cm/hr) and highest for MWD (2.09mm) and for Ksat (49.20cm/hr) were obtained under barefallow and grass treatments respectively. The percentage aggregate size above 2.0mm was highest in grass and lowest in barefallow. Calculations showed significant positive correlation (r = 0.50 at P = 0.05) between organic matter and MWD. There was significant negative correlation (r = -0.60 at P = 0.05) between organic matter and bulk density and significant positive correlation (r = 0.80 at P = 0.05) between organic matter and saturated hydraulic conductivity. The pentades were generally wet during the study periods in the two seasons. Cumulative runoff was highest in barefallow and lowest in sorghum (87mm and 41mm respectively). The highest soil loss of 1.13kg/m2 and relatively low loss of 0.55kg/m2 were obtained in cocoyam and sorghum respectively in 2005. Runoff and soil loss were reduced by 100% under grass and legume. Cocoyam and sorghum reduced runoff by 20% and 53% respectively. Sorghum reduced soil loss by 35%, while there were no differences in the percentage reductions due to barefallow and cocoyam treatments. Runoff as percentage of rainfall was highest in barefallow and lowest in sorghum (60.6% and 19.9% respectively). Erosion rate was lowest in sorghum (0.2kg/m2/month) and highest (0.4kg/m2 month) in cocoyam. Under BF the rate was 0.3kg/m2/month. The mean yield of cocoyam was 1.35t/ha and that of sorghum was 0.88t/ha.
Vegetation degradation is regarded as a reduction in the available biomass and decline in vegetative ground cover. It may result from deforestation and overgrazing. Such decline in vegetative cover is a major contributory factor to soil degradation particularly with regard to soil erosion and loss of soil organic matter (Douglas, 1994). The main factor – directly or indirectly responsible for soil and land degradation process is water erosion (Spaan, 2005). Severe surface erosion is linked with intensive precipitation, high detachability of surface soil materials and reduced infiltration. This is induced by poor and weak soil structure and by poor cover of vegetation or plant residue in critical periods (Pla 1997). Most arable soils of the world suffered from serious problems of degradation due to high rate of runoff erosion (Piccolo et al., 1997). This has posed a great threat to agricultural sustainability as it decreases actual and potential soil productivity (Lal 1998).
In the humid tropical region, the current increase in population has led to intensive cultivation of both low and uphill land, leaving the soil surface exposed to destructive effect of high energy rains with rapid organic matter depletion. In this fragile tropical environment, the extent of bare areas increases and the sustenance of biomass production is reduced (Valentine and Juneau 1989). Combating vegetation degradation either through natural grassland or planted crops has the potential to contribute directly to the maintenance and improvement of soil productivity. Vegetation cover protects the soil from the destructive effects of intense rainfall and detachability of surface materials. It reduces runoff, conserves moisture and retains sediment and organic debris. It also allows drainage of excess water due to their semi-permeable nature (Kiepe 1995).
Conventional tillage, which creates favourable environment for crop growth, can also damage pore continuity and promote dispersion of clay forming crust and create dense, non-friable clods and aggregates. Pagliai (2005) reported that conservation tillage practices such as zero tillage, minimum tillage, surface mulching and contour ploughing reduced run-off and soil loss and were best suited to preventing and controlling crusting. According to Greenland (1981) many soil physical properties became better with zero tillage as compared to intensive cultivation. Zero tillage promotes the activities of soil fauna and improves structural stability.
There are several research works on the influence of tillage on run-off and soil loss in West Africa (Lal 1974, Obi 1982, Obi et al., 1988). However, fewer works have been carried out on zero tillage and on the selection of crops that will provide maximum cover to the soil as well as on expected economic benefit to the farmers. The use of sorghum (sorghum bicolor) and cocoyam (colocasia xanthosoma) to provide immediate soil cover has not been extensively studied in the Southeastern zone of Nigeria. It has become necessary, therefore, to provide information in this regard by identifying the management practices that would protect the soil resource and restore lost productivity.
1.1 Objectives of Study
The study was aimed at evaluating the effects of vegetative covers on physical properties, runoff and erosion in Nkpologu sandy loam soil. The specific objectives include to:
- evaluate the effects of different cover management on properties of the soil.
- determine the amount of runoff and soil loss under each vegetative cover management practice.
- determine cropping practice(s) likely to reduce soil loss to tolerable level.