Field Soil Sampling Density for Variable Rate Fertilization

Abstract

Variable rate fertilizer application being commercially performed today is most often based on a soil test map. The sampling density used to develop a map is often selected without background information regarding field soil test variability. The objective of this study was to determine how many samples should be taken from a field in order to locate and describe major areas of fertility affecting variable rate fertilizer applications. Two 40 acre fields were sampled in an 82.5 ft grid each fall from 1989 to 1992. Soil pH, Bray P1, and available K levels were determined on each sample and maps were made using inverse distance squared estimates. Data were taken from the samplings to represent a 165 ft and 330 ft grid pattern. Maps were developed from these less dense grids and compared with the 82.5 ft grid values. In 1992, a separate 220 ft grid sampling was taken. The 220 ft grid estimates were more highly correlated with the 82.5 ft grid values than were the 330 ft grid estimates, however, membership of 220 ft and 330 ft grid estimates within soil test categories were similar. Fertilizer P and K applications were made in one field following the 1992 sampling. Spring 1993 sampling showed the success of the 220 ft grid in directing a variable rate application of P and K. Comparisons to theoretical P and K applications directed by a 330 ft grid map showed the superiority of the 220 ft grid compared with the 330 ft grid.