ABSTRACT

The effect of sesquioxides and carbon content on soil structural stability was carried out on soil samples collected from four plantation land use systems (cocoa, coconut, oil palm and rubber plantation) at three depths (0-20cm, 20-40cm, 40-60cm) in Edo state. The study also assessed the physicochemical properties of the study soils as well as the relationship between sesquioxides, organic carbon and heavy metals content with depth. Spatial variability of selected properties was assessed with the help of geostatistical map tool and a 4x3 factorial analysis in RCBD was carried on the laboratory data, while FLSD at 5% was used for separation of statistically significant means. The relationships between selected soil properties were assessed by simple linear correlation and multiple linear regression. The result shows that the soils were generally sandy loam, there were no significant differences on percent water stable aggregate (WSA) or mean weight diameter of aggregate for the different land use systems studied. The oil palm land use recorded the highest mean weight diameter of aggregate stability followed by cocoa and rubber while the lowest value was observed in coconut land use but the highest percent water stable aggregate was observed in rubber land use, (85.49%) followed by coconut, (77.41%) oil palm, (76.48%) while the lowest value was observed in cocoa, (74.68%) land use. Both aggregate stability index showed increase with increased soil depth in the different land uses.  Soil pH in H₂O and KCl were highly acidic, the value for pH in H₂O ranged from 4.75-5.22 while in KCl it ranged from 4.22-4.77, and both were significantly different at p<0.05. Organic carbon was low and differed significantly at p<0.05 in the different land uses, rubber plantation soil recorded the highest mean values of 1.76%, while coconut plantation recorded the lowest mean values of 0.88%. Micronutrients (Fe, Cu, Zn, Mn, and B) differed significantly at p<0.05 in the different land uses and also with depth. Oxides of Al and Fe varied from 2.92%- 363% and 4.25%-9.75% at 0-20cm soil depth for Al and Fe respectively. However, Al₂O₃ increased with increased soil depth, while Fe₂O₃ decreased with increased soil depth except in oil palm land use at 40-60cm soil depth. The Al₂O₃ and Fe₂O₃ had a positive significant correlation at p<0.05 with Mg, copper and zinc, while other parameters were not significantly different.  The study showed that sesquioxides and organic carbon had a positive relationship with aggregate stability with coefficient of determination R²=0.35 in cocoa land use, while for coconut, oil palm and rubber plantation, Fe₂O₃ and organic carbon had a positive relationship with aggregate stability with coefficient of determination R²=0.27, R²=0.44 and R²=0.52 respectively in the different land uses. This shows that organic carbon and oxides of Fe contributed more to aggregate stability than oxides of Al. The Al₂O₃ and Fe₂O₃ affected the heavy metal content of the soil as indicated by a high Al₂O₃ coefficient of determination (R²=0.73) for coconut, R²=0.72 for oil palm and R²=0.64 for rubber on the heavy metal while in cocoa land use, with low coefficient of determination (R²=0.37). The Fe₂O₃ coefficient of determination was R²=0.80 for cocoa, R²=0.95 for coconut, R²=0.88 for oil palm and R²=0.69 for rubber land use. The result on spatial variability as shown by the Arc- map indicated that, the physicochemical properties vary highly in the different direction, in the different land uses. Therefore, there is a need for precision agriculture in these land uses in Edo State in order to reduce cost of input such as organic and inorganic fertilizer to achieved maximum output.