Introduction
1. 1 Background of the Study
Soil is one of the world‘s greatest resource. It has been described as a natural body of animal, mineral and organic constituents, differentiated into horizons of variable depth which differ in morphology and physical, chemical and biological characteristics (Joffe, 1948). It is an important natural resource that either directly or indirectly supports most of the planet’s life. A major function of soil on man is that it supports food supply and food security. Soils affect food security directly because it supports agriculture. Agriculture produces the food we eat and provides the primary source of livelihood for 36% of the world‘s total workforce (ILO, 2007). Therefore, threat to soil is threat to food security and economies which rely on agriculture. Food security is directly linked to the ability of the land to support populations (Scherr and Yadav, 1996).
Global concern about soil degradation has grown because of its threat to agriculture and food security. Low agricultural production, food insecurity, low income of the rural population and poverty are consequences of soil degradation (Junge, Deji, Abaidoo, Chikoye and Stahr, 2008). Agcaoili, Perez and Rosegrant (1995) estimated that increasing degradation would lead to as much as 10% decline in productivity in the developing countries and could lead to worsening malnutrition in the developing world.
Some of the challenges posed by soil degradation to food security in the developing countries are; long-term investment and appropriate technology development, dampened economic growth effects of lower farm incomes in irrigated, high quality rain fed and densely populated marginal lands due to lost soil productivity and threat to nutrition and deaths from malnutrition and diseases associated with poor diets (Eswaran, Almaraz, Van den Berg and Reich, 1997; Swift and Shepherd, 2007).
Soil degradation is a loss of soil function. It is a serious and most common form of land degradation because the soil is the basis for production (Blum, 1998). It encompasses physical, chemical and biological deterioration. Examples of soil degradation are loss of organic matter, decline in soil fertility, decline in structural condition, erosion, adverse changes in salinity, acidity or alkalinity, and the effects of toxic chemicals, pollutants or excessive flooding. Soil degradation is the decline in soil quality caused by its improper use, usually for agricultural, pastoral, industrial or urban purposes. A soil is said to be degraded if the loss of basic qualities which manifest in reduced yields, is permanent (Mbagwu, 2003). Soil degradation occurs globally but its negative impacts are most felt in regions which depend solely on agriculture for its income (Swift and Shepherd, 2007). Increased pressure on land as a result of increasing world population has led to increased degradation of world soils. Soil degradation may be exacerbated by climate change.
Soil erosion by water is the commonest way in which soils are degraded and many forms of soil loss are most easily seen during or shortly after periods of heavy rains (Stocking and Murnaghan, 2000). Soil erosion associated with poor land use and management practices is one of the main factors causing degradation (Gessess, Klik and Hurni, 2009). Bridges and Van Baren (1997) observed that the presence of erosion is an indication of an underlying problem. They observed that though soil erosion is a cause and effect of soil degradation, it is easier to treat the cause than the effect. Any negative change in soil properties may lead to decline in soil quality and its ability to resist degradation. Soil degradation processes can occur without human interference but accelerated rates of degradation are often influenced by man‘s intervention in natural processes.
Overgrazing, over cultivation, water logging and deforestation are some of the main causes of soil degradation. Soil degradation is defined by reference to productivity. Studies show that farmers‘perception of declining soil quality are tied to agricultural productivity (Bridges and Van Baren, 1997). Studies also show that soil erosion reduces soil quality (Barrow, 1991; Sanchez, Recalata, Antolin, Carbo‘ and Depaz, 2004). Doran and Parkin (1994) define soil quality as the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental health and promote plant and animal health. Sensitivity and resilience are measures of the vulnerability of soils (Doran and Parkin, 1994; Stocking and Murnaghan, 2000). This vulnerability is affected by factors such as topography, soil properties and human influence (Stocking and Murnaghan, 2000). Soil resilience is defined as the ability of a soil to restore its living systems after disturbance while sensitivity is the degree to which a soil undergoes change due to natural forces, human intervention or a combination of both (Bridges and Van Baren,1997; Stocking and Murnaghan, 2000).
Soil compaction is a major form of soil degradation. It is estimated that soil compaction has resulted in reduction of yields by 20% to 50% in Europe and North America and 40% to 90% in West Africa (Oldeman, Hakkeling and Somboek,1991). Nutrient depletion is another form of soil degradation with serious economic implications for many regions of the world. Experts estimate that soil erosion in Zimbabwe result in an annual loss of P and N (totaling $1.5 billion), while in Asia annual economic loss is estimated at $600 million. In Nigeria, Lal (1995) observed dramatic reductions in maize yields ranging between 30-100% after simulating the effects of soil erosion by artificially removing topsoil on plots. In the same study, yields of cassava also reduced by 14-31% and observations showed that fertilizer amendments did not appear capable of restoring the yields where 10-20cm of top soil had been removed.
Egboka and Okpoko (1984) observed that though the development of gullies in Angulu-Nanka region of Anambra state had been attributed to the influence of human activities on geomorphological processes, observations also showed that the hydrogeological and geotechnical properties of the complex aquifer system underlying the affected areas were also responsible for the degradation of soils in the region. Similarly, Junge (2010) observed that land use intensification led to increased soil erosion and depletion of soil organic matter and nutrients which invariably led to decrease in arable and grazing land, crop production, diary produce and dwindling income for local farmers. Some implications of land scarcity include rural-urban migration, conflicts between land users over scarce productive lands, food insecurity and poverty.
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