CHAPTER ONE
INTRODUCTION
1.1Background of the study
Ever since mankind started agriculture, soil erosion has been the single largest threat to soil productivity and has remained so till date [1]. This is so because removal of the topsoil by any means has, through research and historical evidence, been severally shown to have many deleterious effects on the productive capacity of the soil as well as on ecological wellbeing. Doran and Parkin [2] captioned the impact of soil erosion in their popular maxim that “the thin layer of soil covering the earth’s surface represents the difference between survival and extinction for most terrestrial life.” Although fertile topsoils could be lost when scraped by heavy machineries [3], the key avenues of topsoil loss include water erosion and wind erosion. Sometimes erosion can be such gradual for so long a time as to elude detection in one’s lifetime, thus making its adverse effects hard to detect.
Eswaran et al. [4] propose an annual loss of 75 billion tons of soil on a global basis which costs the world about US $400 billion per year. A review of the global agronomic impact of soil erosion identifies two severity groups of continents and reveals that Africa belongs to the more vulnerable group [5]. Soil erosion by water seems to be the greatest factor limiting soil productivity and impeding agricultural enterprise in the entire humid tropical region [6]. This is evident in many regions of Africa [7], mainly in the humid and 2 Applied and Environmental Soil Science subhumid zones of Sub-Saharan Africa (SSA) where population pressure and deforestation exacerbate the situation and the rains come as torrential downpours, with the annual soil loss put at over 50 tons ha−1 [8].
In Nigeria, the problem is not limited to water erosion as wind erosion prevails mainly in the semiarid and arid zones. For instance, soil loss to wind erosion of 58–80 tons ha−1 has recently been reported from the West African Sahel [9]. Both forms of erosion can thus aptly define land degradation in the region. Soil erosion selectively detaches the colloidal fractions of soils and carts them away in runoff [10, 11]. These soil colloidal fractions (clay and humus) are needed for soil fertility, aggregation, structural stability, and favourable pore size distribution. The concentration of humus is usually higher in topsoils while that of clay is usually higher in subsoils due to illuviation, and this is mostly true in Ultisols that are widespread in Africa. This implies that humus, which has much greater capacity to hold water and nutrient ions compared to clay, its inorganic counterpart [12], is the more easily eroded.