3. RESULTS
The physical and chemical properties of the soils were depicted in Tables 1 and 2. The particle size fractions varied significantly (p <0.05) with the age of plantations, and also varied with depths. The sand fractions which ranged between 698–778 g/kg were higher in the oil palm plantations, while silt (50-80 g/kg) and clay (172-232 g/kg) fractions were higher in the forest (uncultivated) soils. Analyses of the particle size revealed that the soil texture was sandy loam and sandy clay loam. Bulk density (BD) values ranged between 0.93-1.25 g/cm3 and increase with depth. Under both alleys and heaps, the rise in bulk density with depth was noticed, and this could be due to the increasing clay content with depth. In the 0-20 cm layer, soil bulk density levels are relatively lower than those in the 20-40 cm layer.
The soil pH values ranged from moderate to slightly acidic except for the reference soil which was neutral. The pH was higher in the forest surface layer with a value of 7.13 (Table 1) and lowest at the oil palm alley 15-20 years with a value of 5.74 (Table 2). Between the 0-20 and 20-40 cm layers, the results varied significantly. Inside the oil palm alleys, the pH of the soil appears to be lower than that under heaped fronds and fluctuates as the years of planting increase. Generally, available P did show a significant difference across all the oil palm plantations and soil depths at p <0.05. The average available P in the topsoil was higher than in the subsoil. The highest value of the uncultivated topsoil was 6.21 mg/kg, followed by 6.10 mg/kg for the oil palm heaps 15-20 years (Table 1), with the lowest values being 4.43 mg/kg in the oil palm alley 5-10 years (Table 2).
The soil organic carbon (OC) content was moderate, ranging from 12.4 g/kg to 21.7 g/kg for 0-20 cm depth (Table 1) while at 20-40 cm depth, it ranged from 11.6 g/kg to 20.4 g/kg (Table 2). The OC content of the reference (uncultivated) site was significantly (p< 0.05) higher than the cultivated sites. Among the oil palm plantations, the 15-20 years old under heaps has the highest OC content (17.8 g/kg), while the 1-5 years have the lowest OC content (11.6 g/kg). The reference (uncultivated) site’s OC content was significantly (p< 0.05) higher than the sites under cultivation. The OC remained constant for some years after the removal of the current oil palm forest vegetation cover, but after 10 years it was observed that OC accumulation occurred in both alleys and under heaped fronds, especially at a depth of 0-20 cm, aside from that the values observed under heaped fronds were slightly higher. The soil’s total nitrogen (TN) was moderate and ranged from 1.18-2.03 g/kg. The oil palm plantation of 0-5 years and 5-10 years under alley has the same value of 1.18 g/kg at the subsoil. The overall distribution of nitrogen content followed a similar trend to the OC distribution and differed significantly with the age of the plantation.
Exchangeable acidity (EA) was low and fluctuated with the years of oil palm cultivation. No substantial variation was found between the overall EA and the age of the plantations and the depth of the soil at p>0.05. In the uncultivated soil at both depths, the exchangeable bases were higher than in the different age groups of the oil palm soils, both in the alleys and under the palm fronds (Tables 1 and 2). According to the Federal Fertilizer Department’s ranking (2012), exchangeable K is low to moderate in both reference soils and oil palm soils. With respect to the soil depth, the mean average K also showed no noticeable difference but declined with the depth of the soil. The soil’s exchangeable Na is low and the amount seems too small to raise some concerns about any potential physical effect of the soil. Compared to the remaining bases, exchangeable Ca values were greater, followed by exchangeable Mg. In general, exchangeable Mg values initially appeared to increase with time in both alleys and under heaped fronds, but declined gradually with further planting age, while the other bases (Ca, Na and K) fluctuated with the age of the plantations.
Cation Exchange Capacity (CEC) did show significant variation across the plantations at p >0.05, it also significant between the depths (Table 1 and 2). However, at depth of 0-20 cm, the reference soils had the highest CEC value of 6.15 cmol/kg followed by the 0-5 years plantation the under alleys with 5.50 cmol/kg while the 5-10 years plantation under heaps had the lowest value of 5.01 cmol/kg. The soil CEC was low (< 16 cmol/kg) and decreased with depth. There was a general increase in the CEC in both layers after 10-15 years of the alleys and heaped palm fronds to the soil. The percent base saturation (BS %) of the soils was greater than 50%, and the trend is somewhat inconsistent with the plantation age. The values ranged between 85.3% and 93.2% (Tables 1 and 2).