Spatial Variability Status Of Selected Soil Properties In North-East Akwa Ibom State, Nigeria

Spatial variability status of selected soil properties in North-east Akwa Ibom State, Nigeria was assessed. The aim was to establish baseline information on the spatial variability status of selected soil properties in North-east Akwa Ibom State for site -specific and sustainable soil management. A terrain attribute (plan curvature) that is capable of capturing the short-scale spatial variability of soil properties in the field was used to guide field sampling. Plan curvature map was generated from digital elevation model (DEM) of the study area acquired from United State Geological Survey (USGS) at 30m resolution. The plan curvature map was classified into straight, convex and concave surfaces. Modified conditioned Latin hypercube sampling method was used in selecting observation points. Each observation point was purposefully selected to fall within the three classes of the plan curvature map to give a good coverage of both feature space (terrain attributes) and geographical space (study area). Soil samples were collected from each observation point at depths of 0-30 cm and 30-60 cm using soil auger. A total number of 152 soil samples were collected in the study area for laboratory analysis. Data generated were subjected to analysis of variance (ANOVA) and means were separated using least significant difference (LSD) at 5% level of significance. The depth interval of 0-30cm and 30-60 cm were integrated to form depth interval of 0-60 cm and data with skewed distribution were log transformed for semivariogram analysis. Soil properties were then subjected to semi-variogram analysis. The results showed that plan curvature was able to capture short scale spatial variation in some soil properties under study. Soil texture was sand in the surface and subsurface soils of both convex and concave slope while in straight slope, the soil texture was sand in the surface and loamy sand in the subsurface. Soil pH was strongly acidic in both straight and concave slope and slightly acidic in convex slope in both surface and subsurface soil. Organic carbon was high in concave and convex slope and very high in straight topography in both surface and subsurface soils. Total N was low in the convex and concave slope but moderately low in straight terrain. The available P of straight slope was higher than that of concave and convex slope.From the semi-variogram analysis, all the selected soil properties exhibited spatial dependence within some distances. The strength of the spatial dependence varied from moderate for sand, silt, soil pH, organic carbon, total N and available P to weak for clay and strong for exchangeable K. The best fitted models were Exponential for sand and silt; Gaussian for available P and Spherical for clay, pH, organic carbon, total N and exchangeable K.The range of autocorrelation was 136.2 m for sand, 76.4m for silt, 1.6 m for clay, 1.69 m for soil pH, 9.4 m for organic C, 7.1m for total N, 39.2 m for available P and 7.8 m for exchangeable K. This shows that beyond these ranges, the selected soil properties should be managed differently.