SEALING INDEX AND HYDRO-PHYSICAL PROPERTIES OF SHALE AND COASTAL PLAIN SAND UNDER MANAGED FOREST ECOSYSTEMS

The study was undertaken in two forest ecosystems in southern Nigeria to evaluate variability in soil sealing, water retention and structural properties of soils derived from Shale and Coastal plain sand (CPS) under Gmelina and rubber land use. Soil samples were collected at 0-20 and 20-40 cm depths for measurements of particle-size distribution (PSD), pore-size distribution through water desorption procedure, and related hydro-physical properties. Results showed that silt + clay-size fraction was significantly (P=0.05) higher in Shale (>75%) than CPS (<35%). There was a low risk of soil sealing (6.1-6.4%) at 0-20 cm topsoil, and a high risk of sealing (1.4%) at 20-40 cm depth. There was no significant change in sealing index created by the wide contrast between the PSD of Shale and CPS. Water retention at high potentials (-10 to -1500 kPa) were low in CPS, reflecting the influence of a wide inter-particle pore spaces of CPS. Available water (AW), useful available water (UAW) and readily available water (RAW) were significantly higher in Shale than CPS at both 0-20 and 20-40 cm depths (P=0.05). About 24.42% and 53.7% of soil water were released in Shale and CPS, respectively, between saturation (0 kPa) and field capacity (-10 kPa). Aggregate ratio (AR) showed dominance of stable aggregates > 0.25 mm in Shale. There were positive correlation coefficient between mean weight diameter (MWD) and AW (r = 0.486, P=0.05), UAW (r = 0.524, P=0.05) and EAW (r = 0.0635, P=.01). Thus, organic litter from Gmelina and rubber trees can reduce the risk of soil sealing, while micro-porosity dominant in Shale tend to control water retention measurements.