ASSESSMENT OF CHANGES IN SELECTED SOIL PROPERTIES UNDER DIFFERENT LAND USE IN OBAFEMI AWOLOWO UNIVERSITY COMMUNITY , ILE-IFE , NIGERIA

The changes in selected soil properties under different agricultural land use were examined. Six observation sites were chosen at the Teaching and Research Farm, Obafemi Awolowo University, Ile-Ife, Nigeria. Composite soil samples were taken from each of the observation sites. Changes in soil properties under these various agricultural land uses were assessed. Secondary forest; planted fallow with legume; bush fallow with predominantly herbaceous species; the continuous cassava cowpea plot, a plot of continuous maize-soya bean; and a tree crop plantation with Theobroma cacao, selected soil properties assessed were: bulk density, gravel content, organic matter content, soil pH and available phosphorus. The result indicated that the soil pH, organic matter content and available P were significantly lower in soils under the continuous arable cropping compared to planted legume and secondary forest. From the study, the main sources of soil degradation were water erosion, leaching and low level of organic matter. The practice of mixed cropping and cultivation of cover crops over the soil and protection against rain drop and erosional impact, should be encouraged to reduce water erosion and increase both organic matter and available phosphorus content of the soil INTRODUCTION Soil, to different people mean different things.Geologists recognize soil as a product of weathering in relation to its composition or mineral resources while civic engineers take soil to be the materials that supports and influences the behavior of earthen structure such as buildings, bridges, pavements etc. Yet sanitarian knows soil as sewage and waste disposal medium. Whereas, housewife understand soil to be mere dirt. The soil scientists on the other hand perceive soil as unconsolidated mineral matter on the * To whom all Correspondents should be addressed: e-mail: dotmark4great@yahoo.co.uk. Tel: +234 803 395 1110 Amusan et al. EJEAFChe, 5 (1), 2006. [1178-1184] Electron. J. Environ. Agric. Food Chem. ISSN 1579-4377 1179 surface of the earth that has been subjected to and influenced by genetic and environmental factors such as climate, vegetation and topography acting over a period of time and producing a product soil that differs from the material from which it was derived (Aduayi, 1985). Soil properties depend on a number of factors. The factors (parent material, organism, relief, climate and time) though, operating independently, the combine effect over a period of time gives rise to distinct soil type. Knowledge of soil with respect to its properties is of utmost importance in determining the agricultural, engineering or any other use to which it may be put. For instance, soil characteristics such as texture show the proportion of the soil separates. This in turns shows the water holding capacity of the soil. Sanchez (1940) noted that many soil physical properties deteriorate with cultivation rendering the soil less permeable and more susceptible to run-off and erosion losses. The ability of soil to retain water and supply it to plant is one of the main limiting factors in tropical agriculture. The changes in soil that decrease its productivity for crop plant may follow from three processes: cropping, erosion and leaching (Sanchez 1940). This may adversely affect the physical condition or the chemical composition of the soil or both. The soils are made up of materials in three different states – solid, liquid and gaseous. Proper proportion of each of these constituents is necessary if the soil is to be a good medium for growth of plants. Rice and Alexander (1938) noted that the in-organic proportion of the solid material is in size, ranging from stone to gravel down to colloidal particle of clay with diameter of less than one-hundred thousand of an inch the proportion of coarse, medium and fine particles determine the texture of the soil. Rice and Alexander (1938) also noted that coarse and medium materials are comparatively in-active, serving mainly as a supporting frame work of the rest soil. The overall movement of water in the soil is influenced largely by the physical nature of the soil. Aduayi and Ekong (1981) noted that gravelly and sand soils, which have large pores, allow free movement of water without retaining much for plant-use. The soil is a porous body made up of discrete particles of irregular shapes and various sizes. The spaces between these particles form a complicated network of connected cavities of almost every conceivable shape and size, from those that too large for the effective operation of capillary forces to those too small to be seen with a microscope (Aina 1994). The major contribution factor to the increasing rate of soil degradation in West Africa is said to be a shortening of the fallow period in the traditional shifting or bush fallow agriculture (Brown and Flaving, 1988; and Lal, 1973). At high population density, fallowing may be abandoned altogether. (Brown and Flaving, 1988). Attempts at finding out the changes in soil properties under different agricultural changes in land use types inform this study. The main objective of this study therefore, is to examine the changes in selected soil properties, which have been brought about due to differences in land use over a period of time. The soil investigated is coarse texture granite gneiss and were mapped as Iwo Association (Symith and Montgomery, 1962). MATERIALS AND METHODS Study Location The experiment was conducted at the Teaching and Research Farm, Obafemi Awolowo University, IleIfe, lying on approximately latitude 728N and longitude 433E with an altitude of about 244m above mean sea level. The project site falls within the tropical rainforest belt with mean annual rainfall of about 1,350mm and the rainfall pattern is bimodal. The study area and soil sampling points are indicated in Fig. 1. Amusan et al. EJEAFChe, 5 (1), 2006. [1178-1184] Electron. J. Environ. Agric. Food Chem. ISSN 1579-4377 1180 Observation Area A total of six observation sites were selected. The site were: Cacao plantation, a fallow planted with legume (centrosema pubes cence) a bush fallow with predominantly herbaceous weed species (chromolaena odoratum) cassava-cowpea intercropped plot, plot of continuous maize-soyabean cropping and secondary forest. Six soil samples were collected from each experimental site. For the first sample, three core soil samples taken at 0-15cm were collected by soil auger, bulked and carefully mixed before sub-sampled. The same techniques were used for 15-30cm and 30-50cm soil depths.