CHAPTER ONE
BACKGROUND TO THE STUDY
1.1 INTRODUCTION: EFFECTS OF SOIL DEGRADATION ON CROP PRODUCTION: AN ASSESSMENT OF TOTAL NITROGEN AND AVAILABLE PHOSPHORUS IN THE RAINFOREST OF OBIARUKU, DELTA STATE.
Soil is defined as the upper most layer of the earth crust which supports plant growth. Soil is a living entity on which all terrestrial habitats dwell on. Soil over the years has been degraded by environmental degradation. Environmental degradation is a worldwide environmental issue which affects not only the soil but also the natural environment (Diaz and Telleria, 2014). Soil degradation according to Davies, et al., (2010) is the decline in soil quality that can be a result of many factors, especially continuous crop production. Diaz and Telleria (2014), added that soils hold the majority of the world’s biodiversity, and healthy soils are essential for food production and an adequate water supply. Soil degradation also has huge impact on biological degradation, which affects the microbial community of the soil and can alter nutrient cycling pest and disease control, and chemical transformation properties of the soil (Davies, 2009).
The environmental impact of soil degradation on agriculture especially on arable crop production as pointed out by Evans (2016) varies based on the wide variety of agricultural practices employed around the world. Ultimately, the environmental impact depends on the types of crop production and the various types of farm/agricultural practices (such as shifting cultivation, mixed cropping, plantation, bush fallowing etc). It also depends on the system used by farmers. The connection between emissions into the environment and the farming system is indirect, as it also depends on other climate variables such as rainfall and temperature (Evans, 2016). According to him, there are two types of indicators of environmental impact of soil degradation on arable crop production “means based”, which is based on the farm’s production methods, and “effect based” which is the impact that farming methods have on the farming system or on missions to the environmental.
Environmental degradation and loss of ecosystem services will directly affect pests (weeds, insects and pathogens), soil degradation and nutrient depletion, growing conditions through climate and weather, as well as available water for irrigation through impacts on rainfall and ground and surface water (Evans, 2016). These are factors that individually could account for over 50% in loss of crop yield in a given year. The interactions among these variables (soil, land, water, vegetation, forest, etc), compounded by management systems and society, are highly complex. A changing climate will affect evapo-transpiration, rainfall, river flow, resilience to grazing, insects, pathogens and risk of invasions, to mention a few (Diaz and Telleria, 2014). The impact of soil degradation is intensified on all sloping land, where with each degree of slope more of the surface soil is carried away as the water moves downhill into valleys and streams.
The degradation of the natural soil through various human activities such as farm practices causes severe ecological problems and leads to damages of buildings, infrastructures, land degradation and production of sediment (Ndakara and Abotutu, 2010). Indeed soil erosion cause severe ecological degradation and leads to damage of buildings, infrastructures, land degradation and production of sediments (Ndakara and Efe, 2010). This process of soil erosion could be slow and continues unnoticed, or it may occur at an alarming rate causing serious loss of top soil.
Recent studies (such as Myers, 2013; Diaz and Telleria, 2014; Stockings, 2016; Evans, 2016) have shown that the natural ecosystems suffer degradation losses. This is especially evident along stream banks, where degradation occurs naturally due to the powerful action of the adjacent moving water. Increased soil losses occur on steep surfaces when a stream cuts through adjacent land. Even on relatively flat land, stream banks are eroded during heavy rains and flooding (Stocking, 2016). Although soil degradation has been taking place very slowly in natural ecosystems throughout geologic time, its cumulative impacts on soil quality over billions of years have been significant.
Approximately 50% of the earth’s land area is devoted to agriculture: About one-third is planted to crops and two-thirds is grazing land (Myers, 2013). Forests occupy about 20% of the world’s land area. Of these three areas, cropland is most susceptible to degradation because of the frequent cultivation of soils and that vegetation is often removed before the crops are planted which exposes the soil to wind and rainfall energy. In addition, cropland is often left without vegetative cover between planting which intensifies degradation on agricultural lands, degradation rates that are estimated to be 75 times greater than degradation in natural forest areas (Myers, 2013).
Nutrient depletion as a form of soil degradation has a severe economic impact at the global scale, especially in Africa. Stoorvogel, et al. (1993) estimated nutrient balances for 38 countries in Africa. Annual depletion rates of soil fertility were estimated at 22 kg nitrogen (N), 3 kg phosphorus (P), and 15 kg potassium (K) per ha. In Nigeria, soil degradation alone results in an annual loss of Nitrogen and Phosphorus totalling US$1.5 billion. In South Asia, the annual economic loss is estimated at US$600 million for nutrient loss by erosion, and US$1,200 million from soil fertility depletion (Stocking, 2016; UNEP, 2014).
In developing countries, soil degradation is particularly severe on small farms that are often located on marginal lands where the soil quality is poor and the topography is frequently steep. In addition, poor farmers tend to raise raw crops such as corn and beans; raw crops are highly susceptible to degradation because the crop vegetation does not cover the entire tilled soil surface (Stockings, 2016). The prime causes for exposed soil are overgrazing and the removal of crop residues for cooking fuel but even by the 1990s researchers realized that these causes are so intertwined with the effects of rainfall variability and the occurrence of drought on the vegetation that it can be difficult to determine how much erosion is due to human activity (Boardman, 2010). During the degradation process organic matter and essential plant nutrients are removed from the soil and soil depth is reduced. These changes not only inhibit vegetative growth but reduce the presence of valuable biota and the overall biodiversity of the soil (Evans, 2016). These factors interact, making it almost impossible to separate the specific impacts of one factor from another (Boardman, 2010). During soil degradation by rainfall, water runoff significantly increases with less water entering the soil and less water available to support the growing vegetation. Degraded soil carries away vital plant nutrients such as nitrogen, phosphorus, potassium, and calcium. Typically, the degraded soil contains about three times more nutrients per unit weight than are left in the remaining soil.
Once the organic matter layer is depleted, the productivity of the ecosystem, as measured by plant biomass, declines both because of the degraded soil structure and the depletion of nutrients that were contained in the organic matter. In addition to low yields, the total biomass of the biota and overall biodiversity of these ecosystems is substantially reduced (Boardman, 2010). He observed that soil degradation has indirect effects on ecosystems that may be nearly as damaging as the direct effects in reducing plant biomass productivity. Beyond the damage to rain fed agricultural and forestry ecosystems, the effects of soil degradation reach far into surrounding environments (Chambers, et al., 2012).
The environmental impact of soil degradation on agriculture, especially on arable crop production involves a variety of factors from the soil, to water, the air, animal and soil diversity, people, plants and the food itself. Some of the environmental issues that are related to agriculture are climate change, deforestation, genetic engineering, irrigation problems, pollutions, soil degradation, and waste (UN, 2007). Generally, soil degradation and erosion removes soil at approximately the same rate that soil is formed (Boardman, 2010). This result to the low crop yield of agricultural production. However, increased soil degradation at a much faster rate than it is formed is a recent problem and due to mankind’s action which leave the land unprotected and vulnerable coupled with naturally occurring erosive rainfall or wind storms makes soil degradation huge environmental issues. This occurs in both agricultural/farming areas as well as the natural environment (Diaz and Telleria, 2014). It is against this background that this study is carried out to examine the effects of soil degradation on crop production: an assessment of total nitrogen and available phosphorus in the rainforest of Obiaruku, Delta State.
1.2 STATEMENT OF RESEARCH PROBLEM
The major problems associated with soil degradation include loss of farmlands, poor agricultural productivity, loss of live stock breeding, erosional problems, soil infertility, among others. The problems could be stated and discussed below;
Loss of organic matter: Soils may be susceptible to erosion, loss of organic matter leading to poor structure, and pollution by pesticides and heavy metals. Soil erosion has effects external to the farm, through siltation of water courses and transport of pesticides and nutrients. The latter can also be conducted to water via leaching and sub-surface flow. Cultivation systems are among the most important factors influencing soil properties (Boardman, 2010). Both ground and surface waters can be influenced by nutrient and pesticide pollution from arable land. This results in reduction in the quality of drinking water and necessitates expensive treatment. It also has ecological consequences for aquatic life (Stockings, 2016).
Loss of soil nutrients: Another major problem associated with soil degradation is that intensification of farming systems, encouraged by government has exacerbated the problems of soil degradation. Nutrients, especially phosphates cause eutrophication of water, which upsets the ecological balance and can result in undesirable effects such as fish death and algal blooms. Problems are greatest where farming is not intensive and lower in Southern Nigeria. Nitrates are particularly prone to leaching, and concerns over nitrates in water supplies have led to legislation in the form of the Nitrates (Evans, 2016).
Loss of ecosystem: Environmental degradation and loss of ecosystem services will directly affect pests (Weeds, insects and pathogens), soil degradation and nutrient depletion, growing conditions through climate and weather, as well as available water for irrigation through impacts on rainfall and ground and surface water. A changing climate will affect evapo-transpiration, rainfall, river flow, resilience to grazing, insects, pathogens and risk of invasions, to mention a few (UNEP, 2008). Unsustainable practices in irrigation and production may lead to increases salinization of soil, nutrient depletion and erosion. Nutrient depletion as a form of land degradation has a severe economic impact at the global scale, especially in Sub-Sahara Africa. Stoorvogel, et al. (2013) estimated nutrient balances for 38 countries in Sub-Sahara Africa. It was discovered that there is an annual depletion of soil fertility, annual economic loss, nutrient loss all caused by soil degradation and erosion (UNEP, 2014; Stocking, 2016).