COMPARATIVE ANALYSIS OF THE PHYSICAL CHARACTERISTICS OF WATER STABLE AGGREGATES IN SOME FOREST AND CULTIVATED SOILS OF ENUGU STATE, SOUTH EASTERN NIGERIA.
ABSTRACT
A study was conducted to evaluate the physical changes that occur in the soil due to conversion of forest to arable land. Six locations in Enugu State that were selected for this purpose include Ugbo-Okpara (Li), Ugbo-nabo (L2), Ugwogo-Nike (L3), Iyi-Ukwu (L4), Edem (L5) and Ugwo (L6). Soil parameters determined include bulk density, saturated hydraulic conductivity, macro porosity, micro porosity, total porosity, organic carbon, pH, Dithionate citrate bi-carbonate (DCB) Iron oxide, particle size distribution, and micro aggregate stability indices. With respect to the water stable aggregate, the indices used for evaluation include particle size distribution, aggregate size distribution of WSA, mean weight diameter, aggregate stability water retention by WSA at both -33kpa and -1500kpa, available water capacity, organic carbon, aggregated silt plus clay (ASC), water dispersible clay (WDC) and clay dispersion index (CDI). Soils of both land use types were coarse to moderately fine texture, with an average pH of 3.95 and 4.1 in the cultivated and forest land use respectively. In the soil, cultivation significantly (P ≤ 0.05) increased bulk density by 17 %, reduced hydraulic conductivity, macro porosity, micro porosity and total porosity by 38 %, 23 %, 19 % and 21% respectively. Soil organic carbon and Dithionate citrate bicarbonate iron oxide was reduced by 34 % and 6.7 % respectively. Land use had significant effect on aggregaated silt plus clay but no significant effect on water dispersible clay, clay flocculation index and clay dispersion index. However significant effect due to the interaction of land use and location was observed in these micro-aggregate stability indices which imply that variation due to land use was dependent on location. With respect to the water stable aggregates, cultivation reduced the macro aggregates and increased the proportion of the smaller (< 0.25mm) sized aggregates. The mean weight diameter and aggregate stability declined by 21 % and 50 % respectively. Water retention at -33kpa and -1500kpa were not affected by land use while a slight but significant (P ≤ 0.05) influence on available water capacity was observed. Particle size distribution of the WSA was changed with cultivation of forest soils. aggregated silt plus clay and water dispersible clay and SOC declined by 20 %, 15 % and 25 % respectively while the clay dispersion Index increased by 2%. The interaction effects of location, aggregate size and land use demonstrated that cultivation improved the physical properties of the WSA in L1, L2 and L3 while L4, L5 and L6 were adversely affected by forest conversion to crop land. Moreover the magnitude of effect in soil properties due to cultivation varied with location as was observed that the highest decline in soil physical property was indicated in L6 compared to other locations which was attributed to its textural characteristics. Therefore the change or magnitude of change in soil properties was location specific.