4. DISCUSSION
The physical and chemical properties of the soil in different land use may be attributed to differences in the study area’s management practices. Soil texture is an inherent property; spatial variations between soils with less variation associated with cultivation could be due to the textural differences found (Brahene et al., 2016). In the 0-20 cm layer, soil bulk density values were comparatively lower when compared to those of 20-40 cm layer where the content of organic matter was very low. This suggests that OM has contributed significantly in enhancing the soil’s physical properties, thereby contributing to the soil’s structural stability (Germer and Sauerborn, 2008). The BD values obtained cannot hinder root development and penetration.
The low pH of the soils can be attributed to the nature of the parent materials and the high precipitation that causes the basic cations in the soil to leach intensively (Tweneboah, 2000; Owusu-Bennoah et al., 2000, Oyegoke et al., 2017). Low pH values connote the presence of a positively charged colloidal surface capable of attracting negatively charged ions. The values for oil palm production, however, were acceptable because they were below 7.5 and did not favor oil palm production above that value (Okon et al., 2017). Available soil phosphorus level was low, as the values were between 3-7 mg/kg (Federal Fertilizer Department, 2012). This means that the amount of phosphorous is according to the pH status of the oil palm plantations. In this land-use system, the higher available P content within the forest may be correlated with an increase in microbial activity.
The high OC content of soils under heaped fronds is related to the quantity, position, consistency, and temperature and humidity actions on the pruned fronds compared to those under alleys (Kirschbaum, 2000; Comte et al., 2012). As a result of the rapid decomposition of palm fronds, there could be a rise in OC and soil nutrients in planting for less than 10 years, but as the planting age moved to 25 years, the decomposition rate slowed down with decomposed material being covered by overlying palm fronds (Okon et al., 2017). Thus, with the age of the plantation, it is possible to find heaps of different heights (Brahene et al., 2016). Total nitrogen (TN) varies with the quantity of organic matter present in soils, so it has risen without exception, along with improvements in the related status of organic matter. Relatively, TN amount is also determined by organic carbon, which in turn results from plant and root biomass, as well as residues returned to the soil system. The key cause of N deficiency in tropical soils is extreme leaching and erosion due to high tropical precipitation (Aweto and Enaruvbe, 2010; Osinuga and Oyegoke, 2019).
The low level of basic cations in the plantations results from the effect over time of the continuous uptake of nutrients by the plants. The result shows that the intensity of weathering, cultivation, and use of inorganic acid-forming fertilizers affects the distribution of the cations in the soil system and improves their depletion (Owusu-Bennoah et al., 2000). The low values observed for these basic cations may also be due to the low content of organic matter and the presence of low activity clays of the area (Oyegoke, 2011). Subsequently, the obtained CEC is a function of the pH and SOM in the soil and, with the age of the oil palms, the value remained reasonably constant in the soil. This is an indication that the soils remain low in CEC at their natural pH levels, demonstrating the soils’ low nutrient retention ability.