Thesis Abstract

Characterization and evaluation of four toposequences Abstract
Toposequences represent a valuable tool for understanding soil variability and landscape dynamics. In this study, we characterized and evaluated four toposequences in a tropical region to assess the influence of topography on soil properties and land use. The toposequences were selected based on their elevation gradients, ranging from hilltops to valley bottoms. Soil samples were collected at different depths along each toposequence and analyzed for various physical, chemical, and biological properties. The results showed significant variations in soil properties among the four toposequences. Soil texture varied with elevation, with coarser textures found at higher elevations and finer textures at lower elevations. Soil organic matter content was higher in the valley bottoms compared to the hilltops, likely due to the accumulation of organic residues and waterlogging in the lower-lying areas. pH levels also varied along the toposequences, with more acidic soils observed at higher elevations. Land use had a noticeable impact on soil properties within the toposequences. Soils under natural vegetation cover exhibited higher levels of organic matter and microbial activity compared to agricultural soils. Land use practices such as tillage and deforestation were associated with lower soil organic matter content and reduced soil microbial biomass. The toposequences also differed in terms of nutrient availability, with higher concentrations of nutrients such as nitrogen and phosphorus found in soils at lower elevations. Overall, this study highlights the importance of toposequences in understanding soil variability and land use effects in tropical regions. The findings underscore the need for targeted management practices that account for the influence of topography on soil properties. By characterizing and evaluating toposequences, land managers can make more informed decisions regarding soil conservation and sustainable land use practices. Future research should focus on expanding the scope of toposequence studies to different regions and land use types to enhance our understanding of landscape dynamics and soil variability.

Thesis Overview

INTRODUCTION

            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 toposequence  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.