Spatial variability of soil nitrate nitrogen after potatoes and its change during winter

Knowledge of the frequency distribution and spatial structure of the soil NO₃-N is required to develop an efficient sampling strategy. A 1 ha polder field was sampled after the harvest of potatoes in October 1987, and in February and April 1988, without being fertilitzed since March 1987. These data sets were examined by a classical statistical as well as a spatial structure analysis. The October and February data sets were found to be lognormally distributed, the April data showed a normal frequency distribution. All three data sets had a spatial structure, although the October data were anisotropic and needed removal of a trend. The spatial variability of soil NO₃−N decreased, became isotropic and evolved towards a larger range of spatial dependence during the winter. Knowledge of this structure permitted to krige or cokrige the data. The number of samples required to estimate the mean NO₃−N content with an acceptable precision was found to be 39, 43 and 17 in October, February and April respectively.