CHAPTER ONE
INTRODUCTION
1.1 Background of study
Nigerian Sudan savanna zone is situate  between latitude 9 3  and 12 31   N and longitude
- and 14 3 E whi h overs a out 22 8 million he tares representing a out one quarter of Nigeria‟s geographi al area (Odunze, 2006) The region is hara terized y high annual average temperature (28-32°C), short wet season and long dry season (6-9 months), abundant short grasses (<2 m) and a few scattered trees (Lal, 1997). Large expanse of arable land exists in the Sudan Savanna Zone of Nigeria with potential for the production of largely grain crops like maize, sorghum, millet, rice and wheat (FFD 2012). Most Nigerian Savanna soils are highly weathered and fragile with low activity clays, thus making their fertility decline under continuous arable cropping (Odunze 2006; FFD, 2012). Generally, soil productivity declines rapidly when vegetation cover is lost and inappropriate management practices are adopted (Lal 1997), thereby resulting in soil organic matter depletion and reduced agricultural productivity and food security.
Population increase and the need to achieve food security; especially in Nigeria, has given rise to clearing of forests for agricultural land use (Osakwe, 2014). Tropical soils are inherently fragile and therefore, sensitive to land use and management since removal of soil cover and subsequent tillage are activities that are likely to affect soil physical and chemical properties and micro aggregate stability (Osakwe, 2014).
Important soil property to consider for assessing soil quality under forest or cultivated land use is aggregate stability of soil. Soil aggregate is considered as soil quality indicator that provides information on soil‟s ability to function as a as component of the ecosystem (Martinez et al., 2012). Micro aggregate stability may be a good indicator of erodibility of Tropical soils (Igwe and Obalum, 2013). Oguike and Mbagwu (2009) demonstrated that micro aggregate stability measured by aggregated silt plus clay (ASC), Water dispersible clay (WDC), Clay dispersion index (CDI) and Clay flocculation index (CFI) were affected by land use. Soils that have high water dispersible clay are vulnerable to surface sealing, crusting and limits infiltration, thereby increase runoff (Chang et al., 1994). Obi (1982), in an assessment of micro aggregate stability under different land use types in a tropical Nigerian soil, revealed a strong dependence of soil aggregation on land use (Igwe and Obalum, 2013). Gochin and Asgan (2008) investigated effects of land use (forest, pasture and cultivation) on soil quality, and reported 41-89 % less dispersible clay in the forest than in their cultivated counter parts. These studies noted that frequent cultivation caused deterioration of soil quality and enhanced erosion through decreased mechanically dispersed clay. Curtin et al. (1994) demonstrated that higher clay plus silt in water stable aggregates of forest than in Cultivated soils‟ mineralogy and soil organic on appear to influence aggregate stability in Tropical soil but the major micro aggregating agent in tropical soil is iron (Fe) and aluminum (Al) oxides. However, the micro aggregating effect of iron sometime could be masked in soils with relatively high soil organic carbon content (1.6 – 6.9 %) as reported by (Opara, 2009); adding that soil organic carbon can also act as a micro aggregating agent or as a facilitator to micro aggregating effect of iron and Aluminum oxide (Igwe and Obalum, 2013). Soil organic carbon is an important soil property because of its hydrophobic characteristics that has the ability to reduce slaking which precedes dispersion. Mbagwu and Bazzoffi (1998) demonstrated that 70% of variations in
water dispersible particles were accounted for by organic matter. Brubaker et al. (1992) indicated that clay dispersion in water had been found to significantly correlate with total content of clay. Savanna Alfisols are low in inherent fertility, organic matter, cation exchange capacity, and dominated by low activity clays (Odunze, 2003). Savanna Alfisols also support production of crops such as maize, sorghum, millet, cowpea, groundnut, soya beans and cotton, and are cultivated continuously and intensively. This continuous and intense cultivation of soils in the zone has resultant effect of accelerated soil erosion, soil nutrient depletion and soil degradation (Bationo et al., 2003).
Another important physical and chemical soil property controlling solubility of many soil nutrients and influence soil fertility is pH of the soil. For example, Monges et al. (2013) did not find any significant variation in soil pH across land use types while Killic et al. (2012) showed slight change in pH with land use change. Land use affects soilfertility and productivity; this manifests as changes in soil properties such as macro nutrients (Nitrogen, Phosphorous, potassium, Calcium, Magnesium, Sulphur etc), pH, Organic matter, Cation exchange capacity, structure (Birkeland, 1984). Deforestation and cultivation of virgin tropical soils often lead to depletion of nutrients (N, P, and S) present as part of complex organic polymers (Birkeland, 1984).
Soil pH, total N, organic carbon, available P, exchangeable Ca, exchangeable Al, CEC and Al saturation significantly differed with the land use types. Lal (1996) and Shepherd et al. (2000) noted that variation of land use in tropical ecosystems could causesignificant modifications in soil properties. In stressing the effect of this phenomenon on ecosystem, Schipper and Sparling (2000) posited that land use modifications are biologically and chemically more rapid than physical, as forest ecosystems are important ecologically and economically. Forest soils are one of the major sequesters of carbon on earth due to their high OM status (Dixon et al., 1994)
