Spatial variability in topsoil micronutrient contents in a one-hectacre cropland plot

The spatial variability of meso- (Ca and Mg) and micronutrients (Fe, Mn, Cu, Zn, and Co) was investigated in a small cultivated plot (1.8 ha) located in the Ordenes Complex, a district of basic schists in northwest Spain. The topsoil was sampled according to a nested sampling pattern to describe the spatial variability by geostatistical techniques from 0 to about 120 m. At the sampled scale results showed that the determined meso- and micronutrients are spatially dependent; furthermore, pH, organic carbon, cation exchange capacity (CEC) and two texture fractions, namely sand and silt content, also present a distinct ordered spatial variation. The spatial structure of the studied nutrients was analyzed and variogram models with a nugget component and two spatial components with ranges of 20 and 50 m were found to fit the experimental results. Contour maps made by block kriging allowed to identify micro-regions with significantly high or low extractable nutrient contents. Micronutrient distribution appear to depend largely on organic C content, soil texture, cation exchange capacity and pH. The number of samples required to assess a mean value for extractable nutrient content was calculated. Reasonably accurate determination (i.e., 95% confidence and 20% relative error) of mean nutrient contents in a plot of this size and characteristics requires 16 to 20 samples. To reduce error due to spatial dependence, samples should be about 20 - 25 m apart.