SPATIAL VARIABILITY AND SAMPLING STRATEGIES FOR NO3-N, P, AND K DETERMINATIONS FOR SITE-SPECIFIC FARMING

NO3-N, P and K nutrients data were analyzed to examine the extent of s patial variations in their concentration levels and to develop samplin g strategies to obtain their representative mean concentrations and co rrelated characteristics in, spatial domain. The study was conducted a t two farms located within the Coastal Plains of Virginia. The nutrien ts' concentration values varied with coefficients of variation ranging from 13% to 40% at the two sires. Spatial variations along and across the rows at study site were the same at Brandon farm, bur were differ ent at the Renwood farm. The sample size required to determine the rep resentative mean values of NO3-N and K nutrients could be decreased by about 40% to 60% if the associated error in the mean values was incre ased from 5% to 10% level. The concentration values of three nutrients exhibited spatially correlated dependence on their adjacent measureme nts. The correlation structures of nutrients could best be described b y the exponential forms of the semi-variogram model for each site, exc ept for K at Brandon site for which a spherical model was found to be more appropriate. Based on the spatial variability and correlation len gth of nutrients, sampling grids of 32 x 32 m size for NO3-N, P, and K determinations at the Brandon site, and of 18 x 32 m for NO3-N, 29 x 59 m for P, and 23 x 44 m for K determinations at the Renwood site are recommended. However due to cost and practical considerations, sampli ng grid of 18 x 32 m size for the three nutrients is suggested for the Renwood site. A reduction of 12% in P fertilizer at Brandon and of 25 % in K fertilizer at Renwood site could be achieved if fertilizer is a pplied at a variable rate based on she spatial variability of the nutr ient levels of the soil. However the requirement for K fertilizer for Brandon and P fertilizer for Renwood site would be approximately 6% hi gher compared to those for the conventional strategy. The information presented here could be used as a guideline for refining the existing sampling practices for implementing the variable-rate fertilizer appli cation technology. However its economic feasibility is yet to be studi ed.