The spatial variability of some aspects of topsoil fertility in two Belgian fields

In site-specific crop production systems, soil cultivation and crop management activities within one agricultural field are adjusted to the local needs of the soil and the grown crop. These systems require detailed knowledge of the spatial variability of the soil and the soil nutrients in the field. From two fields of 5 and 13 ha located in the main agricultural regions in Belgium, soil profile descriptions, some topsoil nutrient levels (P, K, Na, Ca and Mg), pH and organic matter content were determined on a square grid of 20 x 20 m in February-March of 1990 and 1992. The study of the very mobile nitrogen requires several sampling and analysis campaigns per season, which was too costly at this stage of the research. A considerable variability in soil profile type was found in both fields. The nutrient levels showed coefficients of variation between 5 and 32% with two extremes exceeding 100% for Ca and Mg in the 13-ha field. The spatial structure of the soil nutrients was investigated by calculating their semivariograms to determine the variances of the estimates made by the spatial interpolation technique of 'kriging'. The topsoil nutrients were spatially dependent up to distances of 40-80 m in the 13-ha field. The spatial structure was more complex in the other field where anisotropic semi-variograms were found. The patterns did not change substantially after two years. Management cell size limits for spatially variable field management were derived from the semi-variogram and were between 11 (K and Mg in 1992) and 28 m (Ca in 1990) in the 13-ha field and between 4 (pH in 1992) and 18 m (P in 1990) in the 5-ha field.