PHYSICAL-PROPERTIES OF SOILS OF SOUTHEASTERN NIGERIA AND THE ROLE OF SOME AGGREGATING AGENTS IN THEIR STABILITY

Soils from five representative profiles and 25 additional topsoil samp les from SE Nigeria were used to study physical properties and the rol e of some aggregating agents (such as iron and aluminum oxides, carbon ates, and organic matter) on aggregate stability. The objective was to identify the major intrinsic soil constituents that influence aggrega te stability as well as to evaluate the possibility of using the more easily determinable stability indices to assess the erodibility of the se soils. Aggregate stability was measured at the macro level with the mater stable aggregates (WSA) >0.5 mm and mean weight diameter of wat er stable aggregates (MWD) indices and at the micro level with the WSA , <0.2 mm and the clay aggregation indices. The topsoils were generall y sandy loam to sandy clay loam, with the clay content increasing with depth, Topsoil bulk densities were high, varying between 1.34 and 1.5 5 Mg m(-3). Subsoil bulk densities were also high, with values reachin g as high as 1.56 to 1.92 Mg m(-3) in some horizons. The correlation b etween bulk density and organic matter (OM) was low, but the trend sho wed an increase in bulk density with decrease in OM content. The value s of liquid and plastic limits were low to moderate. The low correlati ons between plastic Limit (PL) and the aggregate stability indices ind icate that PL is not a good predictor of the structural stability of t hese soils. The influence of OM on the aggregate stability indices was more pronounced within than between the soil profiles, implying that the role of OM as an aggregating agent is soil-dependent. The lowest c lay content was obtained with water as the dispersant. Removal of OM a nd carbonates from the soil samples did not produce clay different in amount from that produced when sodium hexametaphosphate alone was used . This shows that OM and carbonates play minor roles as clay aggregati ng agents in these soils, probably because of their low concentrations . Removal of Fe2O3 and Al2O3 with sodium dithionite-citrate-bicarbonat e (DCB) solution produced the highest concentrations of clay in all so ils. This shows that Fe and AZ oxides play the most important clay-agg regating roles in these soils. Therefore, for more reliable and reprod ucible soil particle size analysis, pretreatment for removal of these sesquioxides is needed. The correlations between the soil erodibility factor (K) and the aggregate stability indices were generally low (r < 0.35), implying that these aggregate stability indices are not reliab le for assessing the erodibility of these soils.