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Plant analyses can be a powerful tool in the diagnosis of yield depressions. Critical plant composition data for corn, soybeans, wheat, and alfalfa are presented which can be useful in interpreting plant analyses. Data are also presented to indicate “normal” variations that may be expected to occur in plant composition resulting from variations in varieties, seasons and soil. For the corn plant, the nutrient composition of some elements change with the leaf sampled and the maturity of the plant.

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 PI, 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. Research Question Variable rate fertilizer application is being commercially performed today. Fertilizer application is most often based on a soil test map. The sampling density that is used to develop a map is often picked without adequate background information of field soil test variability. There is little information to suggest how many samples would be necessary from a field to represent the major fertility areas. 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 to describe the major pH, P, and K areas, and then these values were compared in different ways with maps made using less dense grid patterns. Three less dense grids were evaluated to determine which adequately described the original values at the least dense sampling interval. Literature Summary Soil samples have been taken in the past to describe both a field mean and field variability. The greater the sampling density, the more is known of the field variability. The success of a variable‐rate application depends on the ability of the sample grid values to describe the field so that underfertilization and overfertilization are minimized at a reasonable sampling cost. Systematic sampling has been recommended in Illinois for many years, however, recommendations have focused on the need for general variation knowledge, but were not intended to direct a variable rate application. A Wisconsin study has shown that a 200 ft sampling grid may be...

The clay mineralogy of soil samples from the Morrow Plot Experiment (University of Illinois, Urbana Campus) was investigated. Analysis of soil samples taken at various intervals between 1913 and 1996 indicates that there is a significant influence of cropping method on the clay minerals in the soils. Curve decomposition methods were used to identify and follow the evolution of the different clay minerals: mica, illite and two randomly mixed-layered illite-smectite phases. The most striking difference is seen for continuous corn and corn-oats-hay rotations. Little change in clay mineralogy is seen in the rotation plot while a significant loss of illitic material from different phases was noted for the continuous-corn cultivation plots. Use of NPK fertilizer since 1955 appears to restore the clay mineralogy in continuouscorn cropping compared to that of the 1913 samples. From these data it appears that the I-S minerals play the role of a K buffer, becoming K-poor when the soil cannot furnish enough K from mineral reserves of detrital phases and K-rich when the soil is able to release enough K to enter into the I-S minerals, where it is available during a growing season, for plant growth.