Inadequate information on the influence of landscape on soil properties is a major factor limiting agricultural production in Nigeria. In order to characterize and evaluate four toposequences at Eha-alumona, Eastern Nigeria, soil properties were studied. On each toposequence, three profiles were sited one the on crest, mid slope and valley bottom. Standard routine analysis was carried out on the samples collected. The result showed that soil pH was consistently the least variable along the toposequence and at all depths. Highly variable properties on the upper slope (crest) include: CEC, base saturation, exchangeable H2; on the middle slope (hill slope) they were exchangeable K, CEC, base saturation and exchangeable H2; and on the toe slope (valley bottom ) they were available P. CEC, base saturation and exchangeable H2. The moderately variable properties on the middle slope were Ca, Mg, ECEC and N. For the physical properties, clay, silt and total sand were highly variable on the upper slope. Notable differences were found to exist among the various properties evaluated. Variations in soil properties exist and were due to topographic positions and horizon depths. The study also show that the midslope suffers surfacial erosion while the foot slope and the summit do not, instead, they gain organic and inorganic matter enrichment. Most properties were highly to moderately variable, thus, uniform soil management practices along the toposequence should be avoided. The soils of toposequence one were classified under USDA sub-order as Lithic Haplustults (crest), Arenic Fragiustults (mid-slope) and Aquic Kandiustults (toe slope). The soils of the toposequence 2,were classified as Arenic Kandiustults (crest), Arenic Kandiustults (mid-slope) and Arenic Kandiustults (toe slope). For toposequence 3, they were classified as Arenic Haplustults (crest), Arenic Kandiaquults (mid-slope) and Grossarenic Kandiaquults (toe slope). At the upper slope of toposequence 4, the soils were classified as Petroferric Haplustults, mid slope as Typic Kandiustults and the foot slope as Aquic Kandiustults The above soils correlate with Acrisols under FAO-UNESCO Soil Classification. Toposequence one and four were moderately suitable, while toposequence two and three were marginally suitable for arable crop production.
Characterization of soil provides a useful means for understanding soil distribution and variability. The modern soil survey is a fundamental basis for land use planning because it contains both qualitative and quantitative data which enable predictions of many kinds to be made. It aids in correlating and predicting the adaptability of various crops, grasses, and trees, to soils and their behaviour and productivity under different management. Field studies that depict the variability and distribution of soil are panacea for total utilization of a given tract of land. Such understanding enables useful prediction to be made wherever such soils occur making it possible for soils of different parts of the world occurring under similar and different climatic condition to be compared (Buol et al 1980). Soil characterization goes beyond soil testing, it is an integration of both physical and chemical nature of soil. It analysis the inherent characteristics and properties of a given soil with the aim of characterizing them into similar soil units and capability land use units.
Soil suitability evaluation involves characterizing the soil in a given area for specific land use type. The information collected in soil survey helps in the development of land-use plans and to evaluate and predict the effects of the land use on the environment. The suitability of a given piece of land is its natural ability to support a specific land use type. Suitability may be a major kind of land use, such as rain fed agriculture, livestock production, forestry, etc.
As these qualities derived from the land characteristics, such as slope angle and length, and soil texture which are measurable or estimable, it is advantageous to use these latter values to study the suitability. For assessing the suitability of soils for crop production, soil requirements of crops must be known. Also, these requirements must be understood within the context of limitations imposed by land form and other features which do not form a part of the soil but may have a significant influence on use that can be made of the soil (FAO,1976).
Soil classification on the other hand helps to organize our knowledge and facilitate the transfer of experience and technology from one place to another and to compare soil properties. It provides a link between soil characterization and soil survey. According to Lark and Wheeler (2000), variation in soil properties has long been known and had been the subject of much research. It was in recognition of this that Sir Ronald-fisher, and then at Rothamsted, developed a formidable array of statistical methods.
Accordingly, horizons may differ in organic matter content, structure, texture, pH, base saturation, cation exchange capacity as well as many other soil physicals and chemical properties. According to Mullar and Mc Bratney (2001), variability in soil properties at the series level is often caused by small changes in topography that affect the transport and storage of water across and within the soil profile. Hunter et al (1982) and Yost et al (1982) reported that soil-forming factors affect different properties differently at different depths. Variability of soil pH, for e.g. increases with depth (Ogunkunle and Ataga, 1985). Ogunkunle (1993) working on Alfisols of southwestern Nigeria, observed that soil pH was the least variable (low variability) property, irrespective of depth. The variability of properties like organic matter, available phosphorus, total nitrogen and CEC, increases with depth. Properties, such as soil pH and porosity are among the least variable, while those pertaining to water or solute transport are among the most variable. Percentage sand ranges from low to moderate variability. Organic matter and % clay range from moderate to high variability. Available phosphorus and potassium were observed to be highly variable (Jury, 1986 et al, Beven et al, 1993, Wollenhaupt et al, 1997). In general the more variable these properties, the more variable the crop growth and yield. Thus, understanding soil variability is essential in applying location specific (precision-agriculture) management strategies. Therefore, the general objective of this study was to assess the degree of variability of some soil physical and chemical properties along four toposequences for assessing their agricultural potentials.
The specific objectives were to: (i) characterize and classify the soils of four toposequences. (ii) assess the effects of slope characteristics on physico-chemical properties.
(iii) evaluate the agricultural potentials of the four toposequences.