Reducing Analysis Variability of the Illinois Soil Nitrogen Test with Enclosed Griddles

The Illinois soil N test (ISNT) is a simple laboratory analysis that shows promise as a tool to predict if additional N application for corn ( Zea mays L.) will result in a yield increase. Initial testing of the method at Cornell University showed a high degree of day‐to‐day and within‐griddle soil test N variability. At the University of Illinois, similar findings led to modification of the method to include jar rotations 1.5 and 3 h after initiation of the 5‐h incubation period used in the test. To determine if variability could be reduced without additional labor by enclosing the griddles using plywood boxes, experiments were designed to compare soil test variability using both the open‐bench and enclosed‐griddle methods. Enclosing the griddles decreased the coefficient of variance (CV) ( p = 0.013) to 2.5% from 3.8% when analyses were done using the open‐bench method (using 10 exterior positions of the griddle for each method). Enclosing griddles did not impact recovery of added glucosamine‐N but resulted in lower N test values when set at the same temperature as the open‐bench griddles. A comparison study of 14 soils analyzed at the University of Illinois using the standard open‐bench procedure with jar rotation and at Cornell University using the enclosed‐griddle method without rotation showed sample means to be comparable when the enclosed griddles were set at 50.6°C. These results imply that the modified method could be adopted for use beyond well‐controlled laboratory environments but that griddle temperature setting is extremely important.