Spatial variability of some nutrient constituents of an Alfisol from loess I. Classical statistical analysis

Classical statistical techniques were used to evaluate the lateral and vertical variability of some nutrient constituents, viz. NO₃, extractable K and Mg, and organic C, of a loess soil field. In all 50 sites at a 30m × 30m grid were selected within a 10 ha field situated about 25 km west of Hannover. Samples were taken at 0–30, 30–60 and 60–90 cm depths. Analysis based on cumulative probability density function showed that all properties except K in the 30–60 and 60–90 cm depths were normally distributed. K in these depths was lognormally distributed. Mean values of NO₃ increased with depth, that of K decreased and that of Mg remained nearly constant. Comparison of coeffizients of variation (CV – % variation) indicated that organic C was least variable laterally (CV = 8.7) and K the most (CV = 43.8 for 30–60 cm depth). The CV of NO₃ and Mg increased slightly with depth indicating less drastic vertical variation. The number of observations required to yield an estimate of the mean value within a prescribed accuracy and precision is shown to depend upon the nature and extent of the spatial variability of a parameter. Thus, estimated number of samples was smaller for lower CV values and vice versa. The optimum number of samples for a given property is determined by the depth in which the largest estimated number of samples is required to be taken.