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.