Field Evaluation of Two Nitrogen Testing Methods in Maine

Abstract

Although the pre-sidedress soil nitrate test (PSNT) can identify corn (Zea mays L.) fields that may respond to supplemental N fertilizer, the test has not been widely adopted by producers. Additional N-testing methods are needed. Studies were conducted to evaluate the (i) accuracy of a leaf chlorophyll nitrate test for N (LCNT), (ii) influence of hybrid and planting date on chlorophyll readings, (iii) accuracy of normalizing chlorophyll measurements against a high-N reference plot, and (iv) use of early-dent stage earleaf chlorophyll measurements to identify N-deficient or N-excessive sites. Fifty nonreplicated onfarm trials and two replicated experiments were conducted from 1993 through 1995 in central Maine. Following planting, we established high-N subplots (720 sq ft) where we applied 200 lb N/acre. At the V-6 stage, PSNT samples were collected and analyzed using the Penn State quick-test method. A Minolta 502 chlorophyll meter was used to measure V-6 and early-dent stage leaf chlorophyll (SPAD units). SPAD and yield ratios were calculated by dividing whole field and high-N subplot averages. Using the PSNT, we identified sites that would respond to sidedress N 82% of the time. The V-6 stage LCNT was accurate 80% of the time. However, V-6 stage chlorophyll values were influenced by hybrid and planting date. V-6 SPAD ratios may reduce variation, although they did not improve test accuracy on the on-farm trials. Using early-dent stage SPAD ratios, we could identify sites that responded to supplemental N 83% of the time, but we were unable to identify sites with excessive N.