ABSTRACT
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.
CHAPTER ONE
1.0 INTRODUCTION
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.
1.1Â Â Â Â Â Â 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.
CHAPTER TWO
2.0 LITERATURE REVIEW
Human over-population is leading to destruction of tropical forests due to widening practices of slash-and-burn and other methods of subsistence farming necessitated by famine in less developed countries (USDA 1997). A consequence of deforestation is typically large-scale erosion, loss of soil nutrients and sometimes, total desertification.
2.1 Erosion
Brady (1999) defined erosion as the detachment and transfer of soil sediments. Ofomata (1980) defined soil erosion merely as a geomorphologic process, whereby the surface layer of weathered rocks is loosened and a lower horizon in the soil profile is exposed. Soil erosion refers to the gross amount of soil dislodged by raindrops, overland flow, wind, ice or gravity. According to Soil Conservation Society of America (SCSA, 1982) report, soil erosion is also defined as the wearing away of the land surface by running water, wind, ice or other geological agents, including such agent as geological creep.
According to Huypers et al., (1987) there are two main types of erosion namely, natural (geological) and accelerated erosion (man made erosion). Geological erosion is going on all the time and new landscapes are formed, but the process is slow. The amount of soil lost through this way from a hectare is at an average of 1 to 2 metric tons per hectare per year. This removal of soil is often replaced by process of soil weathering. Huypers et al., (1987) were also of the opinion that, through human activities (desertification, slash and burn etc), rain could cause much quantities of soil to be transported. On the average, this is about 50 metric tons per hectare per year.
Lal (1990) also observed that soil erosion by human activity became serious when its rate exceeded threshold value equivalent to the counter-balancing and compensatory rate of new soil formation. The threshold value of erosion – the rate at which it starts depleting soil productivity and causing soil degradation – differs for soils developed on different parent materials and in different climatic regions (SCSA 1982). Accelerated erosion is much more rapid than the geologic erosion and it has severe adverse effects on soil and the environments.
Akamigbo (1984) stated that soil erosion in its various forms had long been recognized as a major impediment to agricultural production in many parts of the world. Soil erosion reduces soil quality and it is a long time problem: globally soil erosion’s most serious impact may be its threat to the long term sustainability of agricultural productivity.
Soil erosion in southeastern Nigeria has become a matter of concern in the past few decades. Gully erosion is a very serious problem in this part of Nigeria.
2.1.1 Factors influencing soil erosion
The rate and degree of soil erosion are influenced by environmental conditions and pedological factors. Giordano et al., (1991) recognized the factors that encouraged soil erosion as removal of vegetation, intensive harvesting, over-grazing, and soil compaction, caused by heavy machinery which reduced hydraulic conductivity and increased bulk density thereby promoting surface water runoff and soil loss.
Wischmeier and Smith (1978) among others recognized some factors which influence erosion as climatic erosivity, erodibility of soil, topography, nature of vegetation (plant cover) and human component. Erodibility is also a product of geology and soil characteristics. The environmental factors can be controlled through human management.
2.1.2 Erosivity
Erosivity is defined as the potential ability of rain to cause erosion (Morgan 1979).
It is a function of physical characteristic of rainfall. Intensity is generally considered to be the most important rainfall characteristic (Morgan 1986 and Gilley et al., 2000).
According to Salles et al., (2000) and Jin et al., (2000) soil loss is closely related to rainfall partly through the detaching power of raindrops striking the soil surface and partly through the stream force of the runoff water. Fournier (1967) remarked that average soil loss per rain event increased with the intensity and duration of the storm.
Lal (1990) added that the time of the peak intensity period in a rain-storm influenced the amount and rate of runoff. He further stated that storms had their highest intensities at the beginning and lowest intensities later. Each intensity distribution pattern presented a different type of erosion hazard. The effect of erosivity can be measured directly if one observes how much erosion is caused by a particular storm or series of storms. The best estimator of soil loss was found to be a compound parameter, the product of the kinetic energy of the storm and intensity (Salako et al., 1991).
- Erodibility
Erodibility is defined as the susceptibility of soil to erosion (Salako, 2003). It is an inherent property of the soil which is influenced by soil properties such as texture, aggregate stability, water transmission characteristics, organic matter content and clay minerals. According to Young (1989), soil erodibility was influenced mainly by changes in soil organic matter content and permeability.
- Topography
Erosion is function of slope and length. Erosion increases with steepness of slope. In western Nigeria, Lal (1976b) observed an increased severity of soil erosion as slope changed from 5% to 15%.
Hudson (1981) showed that steep land was more vulnerable to water erosion than flat land for the obvious reason that erosive forces, splash, scour and transport, all had a greater effect on erosion on steep slope compared to flat land.
Morgan (1979) stated that on flat land surface, rain drops splashed soil particles randomly in all directions while on sloping ground, more soil were splashed down slope than upslope, and the proportion increased as the slope steepened.
- Human factor
Agricultural practices in West Africa involve the destruction of vegetation by clearing land for cultivation (slash-and-burn). The slash-and-burn practice exposes the soil surface to raindrop impact which produces a continuous compacted layer or crust at the surface. The surface crusting would result in decreased water infiltration, increased runoff, poor seedling emergence and often increased erosion (Lal 1979). The replacement of traditional hoe with the plough has increased the disturbance of the topsoil, breaking up its structure and making it less resistant to erosion (Wood, 1992).
Young (1989) observed that a very hot burn (from exposed surface) could oxidize some of the soil organic matter and suppress the microbial activity. This means that the ability of microbes to bind soil particles into aggregate will be hindered. Stocking (1988) showed that vegetation acted in variety of ways by interrupting raindrops, encouraging greater infiltration of water and increasing surface organic matter, thereby reducing the erodibility of the soil.
Hudson (1981) enumerated the purposes of vegetative cover, noting that, it provided the soil with physical protection against scour and reduced the velocity of flow by increasing the hydraulic resistance of the channel, thereby reducing the scouring ability of the flood. Holy (1980) added that the vegetative cover protected the soil surface from the direct impact of raindrops. It enhanced infiltration of rainfall into the soil and reduced surface runoff, thereby improving the physical, chemical and biological properties of the soil.
2.2 Effects of erosion on soil properties
The main agent of soil erosion in the south-eastern Nigeria is water and there are several qualitative reports on the devastation caused to the environment. Mbagwu (1986) noted from the point of fertility depletion and reduction in land productivity that the wide-spread form of sheet erosion was a more serious problem. He concluded that erosion generally resulted in the degradation of the physical, chemical and biological properties of soil and this, in turn, caused drastic reduction in crop yield. The magnitude of yield reduction associated with top soil loss varies with both soil and crop types.
