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 aggregated 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.