Monday, 10 July 2006

Catena/Toposequence/Soil Association: Unifying Concept in Soil Classification, Soil Genesis and Land Use in the West Africa Sub-Region-Nigeria.

Temitope A. Okusami, Obafemi Awolowo Univ, Dept of Soil Science., Faculty of Agriculture, Ile-Ife, Nigeria

The West Africa sub region has a northern limit of 20N (Sahara desert southern front) with southern limit at about 3.50N adjorning the Atlantic Ocean. The longitudinal boundaries are at approximately 17.29W to 15E. In terms of isohyets, it occupies from 100 in the north to 3200 in the south. Nigeria exists within the coordinates 4-14N and 3-15E.Generally, West Africa relief consists of a plain of highlands and broad interior basins but simulating a syncline trough sandwiched between two anticlines. The geological history has been dominated by denundational processes right from post-Gondwana land break-up till Cainozoic Era. The pre-Cambrian (dominant age/rock) consists of crystalline granite and gnessic rocks with landscapes that had been eroded into peneplains. During the tertiary (2-44 Mys BP), the rigid pre-Cambrian rocks got fractured leading to the synclines, an example of such being the Benue trench in Nigeria/Cameroon. The synclines have all along been filled with sediments that are both marine (sea transgression-sandstones, limestones, marls along the coast and into the Benue/Niger trenches in Nigeria) and continental deposits.These scenarios have led to a peneplain (on a small scale) but an undulating landscape (on a large/detail scale). The sequences/series of these landscapes have led to a series of soil-slope relationships tagged toposequences or catena characterized by differential parent materials and uniform parent materials respectively but with identical differentiating topo-drainage characteristics. The toposequence and catena concepts have emanated as slope-soil evolutionary processes. The relative stability of the geology of Africa (Nigeria inclusive and representative of west Africa) has made it possible to develop a consistent hill slope element concepts that could be perturbed only occasionally by tectonic uplifts typical of the events along the Nigeria-Cameroon border in the south-east/southwest flank. This sequence, be it toposequence or catena has come to be known in soil classification/mapping paradigms as soil association. As a classification system, soil association in most countries of sub-Sahara Africa is not taxonomic but nevertheless hierarchical. In Nigeria, soil association belongs to the second highest level followed by the series and what is termed the variants. It is currently the only national scheme of soil classification awaiting full correlation studies and extrapolation of soil series or its modifications across agroecological zones. It has been a vehicle or even a prelude that has stimulated scientific curiosity into landform and soil development theories/paradigms. Globally or at least within the tropical/subtropical world the identification/classification of landscapes as soil associations (land systems) has opened up a vista of challenges to unravel the systematic occurrences of landscapes within climatic/agroclimatic belts. These efforts have been spearheaded by geomorphologists and pedologists working on landforms and the underlying soils. It has led credence or even stimulated the term “pedogeomorphic synthesis” (Gerrard,1981). Milne (1935 ) formulated the catena concept from African landscapes. Ollier (1959) was also at the forefront on the continent of Africa probably followed by others such as Webster (1965), Sparrow (1966), Moss (1968). The genesis and classification of plinthites and petroplinthes (Ferricrete) have also engaged soil scientists as an outcome of the observation of these units within soil associations. The backbone of the understanding of toposequence/catena concept is a function of the different models of slope formation theories. In this context, detailed soil profiles have been strongly related to slope elements. Evidences abound that scientific information on soil-slope relationships within soil association are still on-going, topical and contemporary (e.g., Okusami, 1991). Soil association has been a very veritable tool in agrotechnology transfer within agroecological zones and between transition ecotones. It has effectively acted as a bridge in synthesizing soil survey information in previous works of Vine (1951), Smyth and Montgomery (1962), Obihara et al. (1963), Jungerius (1964), Land Resources Division of The British Ministry of Overseas Development (1966-1976).It has also provided the pragmatic groupings for the needed knowledge in predictions for soils/land suitable for crops such as Cocoa (Theobroma Cacao) (Smyth and Montgomery,1962),Rice (Obihara et al. 1963).etc.

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