2. MATERIALS AND METHODS
Oil palm plantations owned by the Federal University of Agriculture, Abeokuta (FUNAAB), were selected with a reference (control) soil adjacent to the plantations. FUNAAB is located next to Ogun-Oshun River Basin Development Authority (OORBDA), along Alabata road, Abeokuta-Ibadan expressway. It lies in a humid tropical lowland zone with two separate seasons (wet and dry). The wet season runs between March and October, and the dry season runs between November and February. The annual rainfall is between 1000-1500 mm, the annual temperature is between 26-320C, and the relative humidity varies between 70-88%. The University superimposes the Basement Complex’s pre-Cambrian metamorphic rocks with bed-rock consisting mainly of granitic gneisses, horn-blended gneisses, bounded biotite, quartzite, and quartz schists.
The oil palm plantations ranged from young ones aged around five years to farms as old as twenty years at various maturity ages. Marking an area of 30 m by 30 m was followed by sampling. The sampling of farms commenced by establishing four age-based clusters of oil palm plantations into which numerous farms were clustered. A total of twelve parcels, with three parcels for each age group, were chosen as replicates for sampling. Soil samples were obtained at depths of 0-20 cm and 20-40 cm. There were 0-5, 5-10, 10-15, and 15-20 years for the different plantation age groups considered. The sampled plots comprised both alleys within the palm rows and pruned and heaped palm fronds. Undisturbed soil samples were collected within the alleys and under heaped trees for bulk density analysis using core samplers. Under the prunings, especially under old heaps, sampling was conducted with care because the top layer had to be separated from the decomposed material sitting just above it. Soils from the sampled spots of various depths from each plantation were put together to obtain a composite sample. A sub-sample was taken, air dried, crushed, and sieved through a 2 mm sieve for routine laboratory testing, and processed.
Using the hydrometer method (Gee and Or, 2002), particle size composition was performed, bulk density was determined using the core sample method. The pH was calculated electrometrically using the glass electrode pH meter in soil-water suspension, soil organic carbon was measured using the digestion method of wet oxidation (Walkley and Black, 1934) and total nitrogen by the digestion method of macro-Kjeldahl (Bremmer and Mulvaney, 1982). The Bray-1 extractant was used to extract available phosphorus (Bray and Kurtz, 1945), while the P in the extract was determined by the vanado-molybdate blue method (Murphy and Riley, 1962). With 1.0 M KCl and titrated with NaOH, exchangeable acidity (H+ and AI3+) was extracted, and with 1.0 M NH4OAC at pH 7, exchangeable bases were extracted. The atomic absorption spectrophotometer was used to determine Ca2+ and Mg2+, while the flame photometer read K+ and Na+. Cation Exchangeable Capability (CEC) was gotten by the summation of exchangeable bases and total acidity (Chapman, 1965). The base saturation was gotten as the ratio of exchangeable bases to CEC.
Data were analyzed using descriptive statistics to show the relationship between the variables in the plantations. The mean was used to derive the average distribution of the variables, the standard deviation shows how the variables deviate from the mean.