O.A. Folorunso, A.E. Njoku, and P.K. KwakyeDepartment of Soil Science, University of Maiduguri, Maiduguri Nigeria
The Chad Basin of northeast Nigeria has a vast Vertisol area. These soils are not only rich in plant nutrients, but also have a high water-holding capacity, and are therefore capable of supporting large-scale production of food and forage crops. The ability of the soils to support large-scale cultivation of forage crops is particularly significant in the northeast since this area is the leading producer of livestock in Nigeria.
Prediction of soil behaviour and crop productivity is conventionally based on soil test results. Soil samples collected from a field must be representative if test results are to be valid. A procedure to obtain representative soil samples, based on an adequate assessment of the soil spatial variability, is presented.
Surface (0-15 cm) soil samples were collected from 100 locations spaced at 2-m intervals on a transect in the middle of a 2-ha field in the Chad Basin of northeast Nigeria. The samples were analysed for exchangeable cations, saturation extract electrical conductivity, particle size distribution, bulk density, organic carbon and pH.
All the soil properties investigated were normally distributed. Estimated sample autocorrelograms and semivariograms suggested that the soil properties were spatially independent. The implication of the spatial independence of these soil properties in selecting an appropriate soil sampling plan is discussed.
Sample size calculations revealed that while it requires only one sample to estimate the mean values of clay content and pH within ±10% of the population means of these properties at the 95% confidence level, 695 samples would be required to estimate the mean value of exchangeable Mg for the study area at the same level of precision. The implications of these estimates of sample number requirements to obtain reliable soil test results for improved management and enhanced productivity of Vertisols are discussed.
