Field Evaluation of Two Nitrogen Testing Methods in Maine

Although the pre-sidedress soil nitrate test (PSNT) can identify corn (Zeu 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. Research QuestionThe pre-sidedress soil nitrate test (PSNT) is an effective tool to identify corn fields that may respond to sidedress N fertilizer. This tool has not been widely adopted by producers, however. Could a rapid measure of leaf chlorophyll content be as accurate as the PSNT at identifying sites that would respond to N? What factors other than N supply influence the chlorophyll content of corn? If this measurement were as accurate as the PSNT, crop management consultants could test many more fields in the short period prior to sidedress. Literature SummaryThe leaf chlorophyll test has been shown to be effective for identifying N deficient corn fields in limited testing in Pennsylvania, Iowa, Nebraska, Alabama, and Delaware. However, a range of critical chlorophyll values have been reported. Comparing field chlorophyll averages to an N-sufficient subplot could correct some of this variation. Little work has been done to evaluate the chlorophyll content of the earleaf at early dent in order to identify sites with excessive N. Study DescriptionIn 1993 and 1994, 50 fields were selected in a five-county area of Maine to compare the PSNT with the leaf chlorophyll test for N (LCNT). Two replicated experiments were conducted in 1994 and 1995 to study the impact of hybrids and planting dates on leaf chlorophyll content. To create a high-N subplot in the on-farm trials, 200 lb N/acre was applied as ammonium nitrate shortly after plant emergence. At the V-6 stage of corn development, PSNT and LCNT were made. Chlorophyll measurements were taken on the 5th leaf of 30 corn plants with a Minolta 502 SPAD meter in the field, and on 24 plants within the 720 sq ft high-N subplot. Similar chlorophyll measurements were taken on the earleaf at the early-dent stage of development. Fifth-leaf and earleaf SPAD ratios were calculated by dividing the field-average chlorophyll value by the high-N subplot value. Silage yield measurements were collected for the subplot and field area. Relative silage yields were determined by dividing the field average silage yield by the high-N subplot yield for the field, each corrected to 30% dry matter. We conducted replicated experiments to study the impact of planting date and hybrid on V-6 chlorophyll content. We used a randomized complete block with three hybrids and three planting dates in 1994, and two hybrids and three planting dates in 1995. When each planting reached the V-6 stage, chlorophyll measurements were collected from the 5th leaf of 24 plants. Applied QuestionsHow does the V-6 stage leaf chlorophyll test for N (LCNT) compare with the standard PSNT in on-farm trials? The LCNT was as accurate as the PSNT for identifying N responsive sites. Using a critical value of 25 ppm nitrate-N, the PSNT detected sites that would respond to sidedress N 82% of the time. With a critical SPAD value of 41.7, the LCNT, without the use of a reference high-N plot, detected 80% of the N responsive fields we tested. Did hybrid or planting date affect leaf chlorophyll content? Corn planted early in the growing season had generally lower SPAD readings than later planted corn in 1994 and 1995. In 1994, we found that the increase in SPAD values with later planted corn varied among the hybrids tested, but the increase was consistent between hybrids in 1995. In the on-farm plots, SPAD values increased with planting date, but SPAD ratios appears to correct this source of variation. Was the use of a V-6 SPAD ratio more accurate than a V-6 critical level? No. Accuracy decreased about 2% using a V-6 SPAD ratio compared with using the 41.7 critical SPAD value in the on-farm trials. It appears, however, that the SPAD ratio can correct for non-N related variation. Despite the fact that the accuracy was not improved over using a critical level, given the significant hybrid and planting season variation identified in these studies, the need for extra control on environmental and hybrid variation is worthwhile. Can the chlorophyll meter be used later in the season to assess N recom mendations? In this case, the earleaf SPAD ratio at silking was more accurate than a critical value. Using the same SPAD ratio that was used with the V-6 stage test, the earleaf at early dent was accurate 83% of the time at identifying N-responsive sites. The value of late-season testing is that a consultant can get a fast, accurate comparison of the field-average chlorophyll content relative to a high-N subplot and be able to tell the producer that the recommendations were correct without having to do yield checks.