Introduction
Vegetable growers in the Willamette Valley use high rates of nitrogen fertilizers, often exceeding 250 lb actual N/acre/season. While growers believe that these rates are necessary to achieve maximum yields and quality, a considerable portion of the applied fertilizer is not taken up by the crop (Hemphill 1997). This excess has raised concerns that the remaining N may be contributing to nitrate pollution of ground or surface waters. We feel that our research has contributed to an understanding of the yield response of vegetables to N fertilizer and the critical stages for N uptake, particularly as related to residual mineral N after harvest. Both research at the North Willamette Research and Extension Center (NWREC) and a 3-year survey of residual mineral N in grower fields indicate that residual N tends to be greatest following sweet corn crops. For the crops that have been the major focus of our research (sweet corn, broccoli, cauliflower), we feel that more work on rates of fertilizer application, timing, placement,and N source will not lead to meaningful changes in the way we fertilize these crops.
We have recently focused on the most important remaining questions and have moved toward solutions to the problem of high levels of residual applied N following vegetable crops. We have attempted to develop methods to predict crop response to sidedressed N and to determine how much sidedressed N is needed, particularly in sweet corn. Research on silage corn grown with high input of manure indicates that a pre-sidedress soil nitrate test (PSNT) may be useful in determining the amount of additional N needed (Marx 1995). In 1995, we started to extend testing of this method to sweet corn, basing our initial efforts on the results of research in New Jersey (Heckman and Prostak 1992). Our evaluations of the PSNT from 1995 to 1998 (Hemphill 1997, 1999) resulted in a modification of the PSNT that appears applicable to sweet corn production on typical Willamette Valley soils. Starting in 1998, we turned our attention to the possibility that some sweet corn varieties of acceptable yield and quality might require less applied N than does the industry standard variety 'Jubilee.' This might lead to significant reductions in the cost and environmental impact of sweet corn production.
Methods, 1999
'Jubilee,' 'GH 1703,' and 'Supersweet Jubilee' sweet corn were seeded on May 25 on 30-inch rows. The plots had previously been fallow for 2 years. Total N rates of 0, 60, 120, 180, and 240 lb/acre were applied to establish five different levels of soil nitrate. Forty lb N/acre were applied at planting to all but the zero N treatments and the remaining N was applied to the appropriate plots on July 7 (Table 1). All N was applied as urea. Stands varied among varieties, with stand of 'Supersweet Jubilee' being particularly poor. Stands were thinned to at least 6 inches between plants (27,000 plants/acre) in early July. SPAD (Minolta Corp.) chlorophyll meter readings were taken on 10 intact leaves/plot at approximately weekly intervals starting on July 8 until harvest, which occurred on September 7. Plots were sampled for residual soil nitrate on September 15.
Methods, 2000
'Jubilee,' 'GH 1703', 'Supersweet Jubilee,' 'Bonus,' 'Legacy,' and 'Sprint' sweet corn were seeded on June 2 on 30-inch rows. The field was fallow in 1999 and had been used for production of edamame soybean in 1998. Total N rates of 80 and 160 lbs/acre were applied to establish two different levels of soil nitrate. Eighty lb N/acre was applied at planting and the remaining N was applied to the appropriate plots on July 18 (Table 2). All N was applied as urea. Stands varied among varieties, with stand of 'Supersweet Jubilee' again being particularly poor (Table 3). Stands of 'Jubilee,' 'Bonus,' and 'Legacy' were thinned to at least 6 inches between plants in early July. SPAD chlorophyll meter readings were taken on 10 intact leaves/plot at approximately weekly intervals starting on July 27 and continuing until harvest, which occurred on September 7. Plots were sampled for residual soil nitrate during the last week of September.
Results and Discussion, 1999
The highest yield of all varieties was obtained with 120 lb N/acre. Yields tended to decline at higher N rates (Tables 4 and 5). Most of our past trials have produced highest yields at 180-200 lb N/acre, but 120 lb N/acre also produced the highest yields in 1998. As in 1998, with the relatively late planting date, available N in the soil before planting may have been unusually high. This serves to reinforce the importance of the PSNT or other method for predicting sweet corn response to applied N, as high concentrations of available N are occasionally present in Willamette Valley soils, even after high-rainfall winters like those of 1997-1998 and 1998-1999.
Just as in 1998, averaged over all N rates, 'GH 1703' outyielded 'Jubilee' by 0.9 tons/acre (Table 4). However, the difference in yield between these varieties was even greater at the suboptimal rates of N (Table 5). At optimal and supraoptimal N rates, the two varieties did not differ significantly in yield (Table 5). Most of the yield difference at low N was due to differences in mean ear weight, but at zero N, 'GH 1703' also produced more harvestable ears. As in 1998, these two varieties also had a strikingly different appearance in the field. Leaves of 'GH 1703' were darker green and thicker and the color stayed a dark green until harvest, even at the zero rate of N.
Yield of 'Supersweet Jubilee' was much lower than that of the other two varieties, mainly because of poor stands. Ear size for 'Supersweet Jubilee' actually exceeded that of the other varieties, perhaps in part because of less competition among plants (Table 4). In addition to poor emergence, many plants of 'Supersweet Jubilee' failed to develop normally and remained stunted throughout the season.
The first SPAD reading occurred on the day after the second application of N, so all N rates produced about the same reading (Table 6). This was also true 1 week later. But by 2 weeks after N application was completed, differences were evident (Table 6, July 22). In 1998, SPAD readings for treatments receiving suboptimal N had started to decline 3-4 weeks before harvest. This was not the case in 1999 as the decline, averaged over all varieties, was not noted until the last 10 days before harvest (Table 4). SPAD readings for 'Jubilee' did, however, start to decline earlier, as in 1998 (Fig. 1). SPAD readings did not differ greatly between the optimal N rate (120 lb/acre) and higher rates of N (Table 6), indicating that the excess N did not produce greater leaf chlorophyll content or greater leaf N concentrations.
As in 1998, the darker green leaves of 'GH 1703' gave consistently higher SPAD readings than did 'Jubilee,' indicating greater chlorophyll content and, perhaps, greater efficiency in uptake of applied N (Table 6). However, the latter did not appear to be the case (see below). 'Supersweet Jubilee' started out with lower SPAD readings than the other two varieties, reflecting delayed development of plants during the cool June and early July weather. But late in the season, the plants were darker green than 'Jubilee' (Fig. 2.)
As in 1998, soil nitrate concentrations were elevated at time of harvest only for rates of applied N of 120 lb/acre or more (Tables 7 and 8). The residual nitrate-N level of about 20 lb/acre for 'Jubilee' and 40 lb/acre for 'GH 1703' in the surface foot of soil at the optimal N rate of 120 lb/acre is probably acceptable and likely not too much higher than at planting. However, at 180 and 240 lb N/acre, residual nitrate-N varied from 35 to 80 lb/acre. The economic and environmental cost of the use of excessive rates of applied N should be obvious from these data, which are very consistent with those obtained in 1998. Residual ammonium levels did not vary significantly with amount of N applied. Residual soil mineral N levels were not measured in the 'Supersweet Jubilee' plots as the poor stand would have resulted in uptake atypical of a good commercial planting.
We had hypothesized that 'GH 1703,' with its darker green leaves and higher yield potential at low rates of applied N, might leave less nitrate in the soil at harvest, at least at low rates of applied N. This did not, however, appear to be the case (Table 8). Although not statistically significant, 'GH 1703' plots tended to have higher residual nitrate levels.
We had hoped to generate a calibration curve that could be used to indicate N sufficiency at different times during the growing season. If this were still an industry dominated by one variety, this might be a feasible approach. But with several varieties being grown and given that other factors besides availability of N, such as moisture content and other nutrients, can affect leaf chlorophyll content, this approach has its drawbacks. At the very least, calibration curves would be needed for each variety.
We are now advocating the use of a highly fertilized reference strip in a sweet corn field. The remainder of the field could be fertilized according to PSNT or grower experience with that field. Additional fertilizer would be needed if the SPAD reading of the main portion of the field dropped below 97 percent of the reading obtained in the reference strip. For example, if the reference strip reads 55 units, and the rest of the field 53, the percentage would be 96.3 and additional fertilizer should be considered. If the crop is at silking, however, it is unlikely to have time to respond to the additional N. The SPAD reading also does not tell us how much N to add. In no case, however, should the additional application bring the total N applied to more than 200 lbs/acre. The SPAD meter might best be used in combination with the PSNT to ensure that tissue N levels remain adequate.
Results and Discussion, 2000
Averaged over all varieties, higher yield was obtained with 160 than with 80 lb N/acre (Table 3). Of the six varieties, 'Jubilee' had the highest yield, but not significantly higher than 'GH 1703.' Yield of 'Supersweet Jubilee' was lowest, attributable to poor stand. In 1998 and 1999, 'GH 1703' had outyielded 'Jubilee' by 0.9 tons/acre. But of most interest was that only 'Jubilee' and 'Legacy' responded significantly to the higher rate of applied N. This result is consistent with what we have seen with 'Supersweet Jubilee' and 'GH 1703' in previous years and indicates that not all sweet corn varieties require high rates of applied N.
Most of the yield difference between N rates was due to differences in mean ear weight. 'Jubilee' and 'Legacy' produced larger ears at the higher rate of N, but the other varieties had similar sized or even smaller ears at high N than at low N. As in 1998 and 1999, the varieties that did not respond to the higher rate of applied N had a strikingly different appearance in the field. Leaves were darker green and thicker, regardless of rate of applied N, and the color remained dark green until harvest, even at the zero rate of N.
The first SPAD reading occurred 1 week after the second application of N, so both N rates produced about the same reading (Table 10). But by 2 weeks after N application was completed, differences were evident (Table 10, 3 August). SPAD readings for 'Jubilee, 'Legacy,' and 'Sprint' varied with the rate of applied N while SPAD readings for the other varieties did not (Fig. 3). As the season progressed, differences in SPAD readings among N rates also occurred in 'Bonus.' The response of SPAD readings to the higher rate of N corresponded to a higher yield for 'Jubilee' and 'Legacy' but not for 'Bonus' and 'Sprint.'
As in 1998 and 1999, the darker green leaves of 'GH 1703' gave consistently higher SPAD readings than did 'Jubilee,' indicating greater chlorophyll content (Table 10). 'Sprint' also had higher SPAD readings than all other varieties except 'GH 1703' in 2000.
Soil nitrate and ammonium concentrations at time of harvest were higher at 160 than at 80 lb of applied N/acre and the varieties differed greatly in their effect on residual soil nitrate concentration (Tables 11 and 12). The amount of residual N appeared to be related to the stand of the six varieties rather than to any inherent difference in nitrate uptake capacity. The three varieties with optimal stands ('Jubilee,' 'Bonus,' 'Legacy') had very low levels of residual soil nitrate. Residual soil ammonium concentration did not vary significantly with variety (Table 11).
As in 1999, varieties with darker green leaves did not leave less nitrate in the soil at harvest (Table 12). Again, this may be related to the relatively poor stand of some of these varieties, but the results are consistent with those seen in 1999 when 'GH 1703' plots tended to have greater residual nitrate levels even with an optimal stand. If this trend is confirmed in future research, it will mean that one must be careful not to overfertilize these high-chlorophyll varieties.
Literature Cited
Heckman, J.R., and D. Prostak. 1992. Presidedress soil nitrate test (PSNT) recommendations for sweet corn. Rutgers Cooperative Extension and N.J. Agricultural Experiment Station FS 760, Rutgers University, New Brunswick, N.J..
Hemphill, D.D., Jr. 1997. Vegetable research at the North Willamette Research and Extension Center, 1995-1996. Oregon Agricultural Experiment Station Special Report No. 975, Oregon State University, Corvallis.
Hemphill, D.D., Jr. 1999. Vegetable research at the North Willamette Research and Extension Center, 1997-1998. Oregon Agricultural Experiment Station Special Report No. 1000, Oregon State University, Corvallis.
Marx, E. 1995. Evaluation of soil and plant analyses as components of a nitrogen monitoring program for silage corn. M.S. Thesis, Oregon State University, Corvallis.
Table 1. List of treatments in sweet corn experiment, NWREC, 1999. Trt. Variety Total N N at Sidedress applied planting N ---------lb/acre----------- 1 Jubilee 0 0 0 2 GH 1703 0 0 0 3 SS Jubilee 0 0 0 4 Jubilee 60 40 20 5 GH 1703 60 40 20 6 SS Jubilee 60 40 20 7 Jubilee 120 40 80 8 GH 1703 120 40 80 9 SS Jubilee 120 40 80 10 Jubilee 180 40 140 11 GH 1703 180 40 140 12 SS Jubilee 180 40 140 13 Jubilee 240 40 200 14 GH 1703 240 40 200 15 SS Jubilee 240 40 200 Table 2. List of treatments in sweet corn variety x N rate trial, NWREC, 2000. Trt. Variety Total N N at Sidedress applied planting N ---------lb/acre----------- 1 Jubilee 80 80 0 2 SS Jubilee 80 80 0 3 GH 1703 80 80 0 4 Bonus 80 80 0 5 Legacy 80 80 0 6 Sprint 80 80 0 7 Jubilee 160 80 80 8 SS Jubilee 160 80 80 9 GH 1703 160 80 80 10 Bonus 160 80 80 11 Legacy 160 80 80 12 Sprint 160 80 80____ Table 3. Main effects of rate of applied N and cultivar on stand and yield of sweet corn, NWREC, 2000.___ Seedlings/ Yield No. of Mean ear 30 ft (tons/acre) ears/plot wt (g) N rate (lb/acre) 80 58 7.6 45 242 160 59 8.9 50 255 Significance NS ** ** NS Cultivar Jubilee 70 9.5 53 251 SS Jubilee 18 6.7 35 270 GH 1703 46 9.4 49 274 Bonus 113 7.6 55 199 Legacy 74 8.5 54 226 Sprint 34 7.8 42 271 Significance *** * *** ** LSD (0.05) 8 1.5 6 40 ***,**,*,NSSignificant at P = 0.1, 1.0, and 5.0 percent levels and nonsignificant, respectively. Table 4. Main effects of rate of applied N and cultivar on yield of sweet corn, NWREC, 1999.___ Yield No. of Mean ear (tons/acre) ears/plot wt (g) N rate (lb/acre) 0 5.7 38 234 60 8.0 49 269 120 9.7 55 277 180 9.1 53 270 240 9.1 53 272 Significance *** *** * Cultivar Jubilee 8.3 54 239 GH 1703 9.2 53 271 SS Jubilee 7.3 41 279 Significance *** *** *** LSD (0.05) 1.0 5 10 ***,*,NSSignificant at P = 0.1 and 5.0 percent levels, and nonsignificant, respectively. Table 5. Interaction of rate of applied N and cultivar on yield of sweet corn, NWREC, 1999. Cultivar Yield No. of Mean ear N rate (lb/acre) (tons/acre) ears/plot wt (g) 0 Jubilee 4.5 37 187 GH 1703 7.3 46 247 SS Jubilee 5.3 30 269 60 Jubilee 7.7 53 229 GH 1703 8.9 52 267 SS Jubilee 7.4 41 287 120 Jubilee 10.5 62 268 GH 1703 10.1 58 274 SS Jubilee 8.5 46 288 180 Jubilee 9.9 60 261 GH 1703 9.8 55 277 SS Jubilee 7.6 44 272 240 Jubilee 9.1 57 249 GH 1703 10.1 55 288 SS Jubilee 8.0 45 279 LSD (0.05) 1.7 8 42 Table 6. Main effects of rate of applied N and cultivar on SPAD chlorophyll measurements in sweet corn, NWREC, 1999.______________________________________ Date of SPAD measurement 8 July 15 July 22 July 30 July 5 Aug. 12 Aug. 24 Aug. 1 Sept. N rate (lb/acre) 0 32.1 35.0 36.8 37.3 39.3 41.2 41.5 44.4 60 34.2 37.0 40.2 43.3 48.2 51.1 49.5 48.8 120 32.8 35.4 41.3 44.9 48.9 53.2 53.0 53.4 180 32.8 35.8 41.8 45.9 50.8 53.5 54.7 54.3 240 31.7 34.5 42.1 45.0 50.4 53.3 54.1 55.4 Significance NS NS *** *** *** *** *** *** Cultivar Jubilee 32.3 35.2 37.6 38.7 42.9 44.6 42.7 42.1 GH 1703 36.1 39.9 45.1 48.1 53.1 56.7 58.1 59.2 SS Jubilee 29.7 31.5 38.7 43.0 46.5 50.1 50.9 52.6 LSD (0.05) 1.1 1.3 1.2 1.6 2.2 3.0 2.7 1.8 Significance *** *** *** *** *** *** *** ***_ ***,NSSignificant at 0.1 percent level and nonsignificant, respectively. Table 7. Main effects of rate of applied N and cultivar on post- harvest soil nitrate and ammonium concentrations, NWREC, 1999. Soil nitrate-N (ppm) Soil ammonium-N (ppm) Cultivar Jubilee 7.0 1.5 GH 1703 10.8 1.4 Significance NS NS N Rate (lb/acre) 0 1.0 1.5 60 2.0 1.4 120 6.0 1.5 180 14.9 1.4 240 19.6 1.5 Significance *** NS ***,NSSignficant at 0.1 percent level and nonsignificant, respectively. Table 8. Interaction of rate of applied N and cultivar on post-harvest soil nitrate and ammonium concentrations, NWREC, 1999. Cultivar N rate (lb/acre) Soil nitrate-N (ppm) Soil ammonium-N (ppm) Jubilee 0 0.7 1.3 60 1.1 1.3 120 4.4 1.7 180 9.4 1.6 240 19.3 1.7 GH 1703 0 1.5 1.6 60 2.8 1.5 120 11.8 1.3 180 20.4 1.3 240 19.9 1.0 LSD (0.05) 9.0 NS Table 9. Interaction of rate of applied N and cultivar on yield of sweet corn, NWREC, 2000. N Rate Cultivar Yield No. of Mean ear (lb/acre) (tons/acre) ears/plot wt (g) 80 Jubilee 7.8 51 217 160 11.1 56 286 80 SS Jubilee 6.4 33 272 160 7.1 38 269 80 GH 1703 9.1 46 282 160 9.7 52 267 80 Bonus 7.5 54 197 160 7.7 55 201 80 Legacy 7.4 49 215 160 9.7 59 237 80 Sprint 7.3 39 269 160 8.3 44 272 LSD (0.05) 2.2 9 53 Table 10. Main effects of rate of applied N and cultivar on SPAD chlorophyll measurements in sweet corn, NWREC, 2000._______________ Date of SPAD measurement 27 July 3 Aug. 10 Aug. 17 Aug. 24 Aug. 1 Sept. N rate (lb/acre) 80 45.7 44.9 46.4 40.6 40.5 40.1 160 46.2 47.3 48.8 44.7 45.4 45.5 Significance NS * * ** *** *** Cultivar Jubilee 44.6 42.6 42.9 38.4 37.8 37.5 SS Jubilee 42.6 43.6 46.6 43.8 44.0 42.6 GH 1703 50.8 53.8 57.2 50.5 52.3 53.8 Bonus 43.4 41.5 41.1 37.3 35.1 36.1 Legacy 43.1 42.4 43.3 36.4 36.4 35.7 Sprint 51.1 52.9 54.8 49.6 52.0 50.9 LSD (0.05) 2.5 1.9 2.8 3.5 4.1 4.6 Significance *** *** *** *** *** *** ***,**,*,NSSignificant at 0.1, 1.0, and 5.0 percent level, and nonsignificant, respectively. Table 11. Main effects of rate of applied N and cultivar on post- harvest soil nitrate and ammonium concentrations, NWREC, 2000. Soil nitrate-N (ppm) Soil ammonium-N (ppm) Cultivar Jubilee 2.2 1.9 SS Jubilee 11.8 3.0 GH 1703 16.2 2.5 Bonus 4.9 5.0 Legacy 3.5 2.9 Sprint 21.6 6.6 Significance *** NS N Rate (lb/acre) 60 6.4 2.3 180 13.7 5.0 Significance ** ** ***,**,NSSignficant at P = 0.1 and 1.0 percent, and nonsignificant, respectively. Table 12. Interaction of rate of applied N and cultivar on post-harvest soil nitrate and ammonium concentrations, NWREC, 2000. Cultivar N rate (lb/acre) Soil nitrate-N (ppm) Soil ammonium-N (ppm) Jubilee 80 1.6 1.8 160 2.8 2.0 SS Jubilee 80 8.4 1.7 160 15.2 4.3 GH 1703 80 6.0 1.7 160 26.4 3.4 Bonus 80 3.7 3.4 160 6.0 6.6 Legacy 80 2.6 2.2 160 4.6 3.7 Sprint 80 16.1 3.0 160 27.1 10.2 LSD (0.05) 9.5 4.0