A Field Exercise to Acquaint Students with Soil Testing as a Measure of Soil Fertility Status and Field Variability

Abstract

A laboratory exercise was developed for a senior-level soil fertility class to acquaint students with the effect of field variability on uses of soil testing. Student pairs took 25 random surface (0–15 cm) soil samples from a 2500 m2 sampling area, from four 25 by 25 m sections within the sampling area, and single point samples from the center of each of 25, 5 by 5 m grids of one of the sections. Each set of 25 random samples were combined, mixed, and the composite analyzed as a single sample. Sample results were compared between pairs of students sampling the same area. Also, split samples from the same composite sample were sent to the laboratory to determine the variability within the laboratory. Soil samples were analyzed for pH, buffer index (BI), nitrate-nitrogen (NO3–N), phosphorus (P), and potassium (K). Results were given to students to assess what effect different soil sampling procedures had on soil test results and how this would influence fertilizer and lime recommendations. The number of subsamples needed to obtain a good composite was between 15 and 20, based on results of this excercise. Surface contour maps, developed from the results of grid sampling, were used to illustrate the variability of the nutrients throughout the field. Sampling and travel to and from the off-campus site were easily completed in a 2-h lab period. This exercise promoted student interest in field work and improved their understanding of soil testing and fertilizer recommendations.