THE SPATIAL VARIABILITY OF SOME ASPECTS OF TOPSOIL FERTILITY IN 2 BELGIAN FIELDS

In site-specific crop production systems, soil cultivation and crop ma nagement activities within one agricultural field are adjusted to the local needs of the soil and the grown crop. These systems require deta iled knowledge of the spatial variability of the soil and the soil nut rients in the field. From two fields of 5 and 13 ha located in the mai n agricultural regions in Belgium, soil profile descriptions, some top soil nutrient levels (P, K, Na, Ca and Mg), pH and organic matter cont ent 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 t his 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 C a and Mg in the 13-ha field. The spatial structure of the soil nutrien ts was investigated by calculating their semivariograms to determine t he variances of the estimates made by the spatial interpolation techni que of 'kriging'. The topsoil nutrients were spatially dependent up to distances of 40-80 m in the 13-ha field. The spatial structure was mo re complex in the other field where anisotropic semi-variograms were f ound. The patterns did not change substantially after two years. Manag ement cell size limits for spatially variable field management were de rived from the semi-variogram and were between 11 (K and Mg in 1992) a nd 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.