Tuesday, November 6, 2007
223-11
Modification of the Illinois Soil N Test: Adaptation for Routine Analysis.
Shelly Shultz, B44N Smyth Hall, Virginia Tech, Virginia Tech University, Crop & Soil Environmental Sciences- 0404, Blacksburg, VA 24061, John Spargo, Smyth Hall, Virginia Tech, Virginia Tech University, Crop & Soil Environmental Sciences- 0404, Blacksburg, VA 24061, and Marcus Alley, 416 Smyth Hall, Virginia Tech, Virginia Tech, Crop & Soil Environmental Science, Blacksburg, VA 24061-0403.
Quantification of organic N mineralization during the corn growing season should improve the precision of fertilizer N recommendations. The Illinois soil N test (ISNT) has shown promise 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 hydrolysable N by treating 1 g of soil with 10 mL of 2 M NaOH in a 473-mL wide-mouth Ball® jar, and heating for 5 hrs at 50o C on a hotplate to liberate [NH4++amino sugar] – N as gaseous NH3 which is collected in H3BO3 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 a hotplate 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 50o C for 5, 6, 7, 8, 9, 10 or 15 hrs. Soil samples were also analyzed with the un-modified method for comparison. All determinations were conducted in triplicate. Use of an incubator set to 50o 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 to the unmodified method (CV = 4.3 vs. 7.4, respectively). Modifying the ISNT by using an incubator instead of a hotplate increases measurement precision and allows for greater sample throughput.