Modification of the Illinois soil nitrogen test to improve measurement precision and increase sample throughput

Quantification of organic N mineralization during the corn (Zea mays L.) growing season should improve the precision of fertilizer N recommendations. The Illinois soil N test (ISNT) has shown promise in some regions as a useful tool for estimating mineralizable soil N; however, the procedure needs to be modified for use in routine soil testing labs that must process hundreds of samples per day. The assay determines alkali hydrolyzable N by treating 1 g of soil with 10 mL of 2 mol L^-1 NaOH in a 473-mL wide-mouth Ball jar, and heating for 5 h at 50°C on a hot plate to liberate (NH₄⁺ + amino sugar)-N as gaseous NH₃, which is collected in H₃BO ₃ solution and subsequently determined by acidimetric titration. The objectives of this study were to determine if variance in measurement values could be reduced and sample throughput increased while maintaining accuracy by using an incubator to replace the hot plate as the heat source. Thirty-five soils collected from N-response trials in Virginia were used in this study. Jars were heated in an incubator at 50°C for 5, 6, 7, 8, 9, 10, or 15 h. Soil samples were also analyzed with the unmodified method for comparison. All determinations were conducted in triplicate. Use of an incubator set to 50°C reduced the total recovery of N from the samples, but increasing the diffusion period increased N recovery. The 15-h diffusion period resulted in quantitative recovery of ISNT-N with significantly (P < 0.05) improved measurement precision compared with the unmodified method (CV = 4.3 vs. 7.4, respectively). Modifying the ISNT by using an incubator instead of a hot plate increases measurement precision and allows greater sample throughput.