Data presentation, interpretation, and communication

Abstract

The majority of studies on soil fertility use experimental and statistical procedures which require statistically independent samples and uniform experimental plots. This hypothesis can only be satisfied and verified in practice with geostatistical analysis. The objective of this study was to assess and analyse the effect of land use on the spatial variability of extractable soil nutrient (Ca, Mg, K, Fe, Mn, Cu and Zn) determined by routine techniques on two adjacent fields with contrasting land uses. Point samples were collected on the surface horizon of a 190 m plot, which was half cultivated and half on natural vegetation. Samples were taken on a regular 2 m x 2 m grid, with 35 points on the natural vegatation and 30 points on the cultivated area. The coefficients of skewness and kurtosis indicate that extractable nutrients didn't show normal distributions in all cases. Mean nutrient contents, with the exception of Fe, were higher in the cultivated than in the natural plot, illustrating the homogenizing and diversity‐reducing effects of cultivation. Data were also analyzed by geostatistical techniques. The semivariogram show that the extracted nutrient contents did not have any variance structure (pure nugget effect), in both cultivated and natural areas. Thus, samples collected at 2 m distance can be considered as independent experiments in this area. The number of samples required to assess a mean value for extractable nutrient contents was calculated.