SPATIAL-ANALYSIS OF SOIL FERTILITY FOR SITE-SPECIFIC CROP MANAGEMENT

Spatial patterns of soil properties and nutrient concentrations need to be characterized to develop site-specific farming practices that match agricultural inputs with regional crop needs. The spatial variation of soil organic C (SOC), soil water content (SWC), NO3-N, PO4-P, and K were evaluated in the 0- to 15-cm layer of a 3.3-ha field (Typic Haplaquoll and Argiaquic Argialboll) cropped with maize (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The range of spatial correlation was determined from semivariance analyses of the data and was found to vary among and within fertility parameters. Nitrate had the shortest correlation range (<5 m) and SOC had the longest (>180 m), whereas SWC, PO4-P, and K had intermediate spatial correlation ranges. In addition, SOC was found to have small-scale spatial variation nested within large-scale spatial variation. The spatial pattern of NO3-N changed with time. Frequency distributions of SOC and SWC were close to normal, whereas the distributions of NO3-N, K, and PO4-P data were skewed. Median polishing detrending and trimming of outlying data were useful methods to remove the effects of nonstationarity and non-normality from the semivariance analysis. The results suggest that reducing sampling intervals from 50 to 1 m would reduce the variance of SWC, SOC, NO3-N, PO4-P, and K estimates by 74, 95, 25, 64, and 58%, respectively. A useful sampling pattern for characterizing the spatial variation of several soil properties-nutrients and scales should be random with sample spacing as close as 1 m and as far apart as the longest dimension of the field.