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.
1.1 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.
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
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.
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.
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.
EFFECTS OF COVER MANAGEMENT PRACTICES ON PHYSICAL PROPERTIES, RUNOFF AND EROSION IN NSUKKA SANDY LOAM SOIL
