Physicochemical properties of earthworm casts and uningested parent soil from selected sites in southwestern Nigeria

Earthworms modify both the physical and chemical properties of soils. In a study on the possible modification of soil properties by earthworms, earthworm casts and uningested A and B soil horizons from three sites in southwestern Nigeria were analysed for selected physical and chemical properties. The casts were derived from the earthworm species Hyperiodrilus africanus. Results were analysed by a combination of analysis of variance with comparison of means and correlation analysis. Earthworm casts were significantly enriched in exchangeable bases, organic matter, base saturation and cation exchange capacity (CEC). The light fraction organic matter (LFOM) (density < 1.75 g cm−3) was more concentrated in the A horizon while the polysaccharides attached to heavy fraction (HFPS) (density > 1.75 g cm−3), presumably composed mainly of the exudates of soil microorganisms, were significantly highest in the casts. The clay and sesquioxide contents differed significantly among the groups analysed in the order B horizon > casts > A horizon, while the sand content ranged in the order A horizon > casts > B horizon. However, the fine sand and silt soil fractions were significantly highest in the casts. The crystalline sesquioxide contents were highest in the B horizon, while the amorphous Fe and the sesquioxides associated with organic complexes were higher in casts. There was no significant difference in the soils’ content of dispersible clay, while the wet stability of >250 μm aggregates from the B horizon was less than those from casts and the A horizon. Macroaggregate stability in casts was positively correlated with LFOM, amorphous Fe and Al, while microaggregate stability was positively correlated with HFPS. Meanwhile, microaggregate stability in the soil B horizon was mainly due to the crystalline forms of Al and Fe.