Introduction
The possibility that the high rates of N needed for maximum yield and quality of vegetable crops are contributing to nitrate contamination of groundwater is a major concern in Oregon's Willamette Valley. We feel that our research has contributed significantly to an understanding of the yield response of vegetables to N fertilizer and the critical stages for N uptake, particulary as related to residual mineral N in the soil after harvest. For the crops of our major efforts (sweet corn, broccoli, cauliflower), we feel that more work on rates of fertilizer application, timing, placement, and N source will not lead to significant changes in the way we grow these crops.
Our focus now is on the most important remaining questions and movement toward possible solutions to the problem of high levels of residual applied N following vegetable crops, particularly sweet corn. We are attemping 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 inputs of manures or other organic sources indicates that a pre-sidedress soil nitrate test (PSNT) may be useful in determining the amount of additional N needed. Some of this work with silage corn has been done in Oregon. We are interested in extending this avenue of research to sweet corn in Oregon, basing our initial efforts on results of PSNT research on sweet corn in New Jersey.
Methods
NWREC plots, 1995
'Jubilee' sweet corn was planted on 30-inch row spacing on 16 May. Forty pounds N/acre as 10-20-20 was broadcast and disked into the entire area before planting. Nitrogen as urea was applied at four rates (0, 40, 80, 120 pounds/acre) at planting. Before application of mid-season sidedress N on 21 June, a pre-sidedress soil nitrate test (PSNT) sample was collected from the surface foot of soil and analyzed for nitrate- and ammonium-N (Table 3). Sidedressed N rates of 0, 40, or 80 pounds/acre were superimposed on each of the initial N rate treatments, resulting in total N rates ranging from 40 to 240 pounds/acre. On 7 July, SPAD chlorophyll meter readings were taken and leaf samples collected and analyzed for total N content. At harvest, stalk samples were collected from six treatments (Table 4, first three and last three treatments) and analyzed for nitrate-N concentration. Corn was harvested from 40 row feet near the center of the plot on 18 August. Following harvest, soil was sampled from the 0-1 and 1-2 foot depths and analyzed for nitrate- and ammonium-N content.
NWREC plots, 1996
'Jubilee' sweet corn was planted to 15 x 30-foot plots on 30-inch row spacing on 26 May. At planting, N (as urea) was applied at rates of 0, 40, 80, and 120 lb/acre to establish four levels of soil mineral N. Before applying mid-season sidedressed N, a PSNT sample was collected from the surface foot of soil and analyzed for nitrate- and ammonium-N (Table 5). At the same time, 10 leaves from each plot were collected for analysis of leaf N content and chlorophyll meter (SPAD) readings were taken on 10 additional intact leaves/plot. Additional N applications of 0, 40, and 80 lb/acre were then superimposed on each of the initial treatments, resulting in total N rates of 0 to 200 lb/acre. Three weeks after the second N application another set of SPAD readings were made. Corn was harvested from 40 row feet on 29 August. Following harvest, soil was sampled for determination of residual nitrate and ammonium content.
On-farm PSNT Trial, 1995
Sixteen experiments were conducted on six farms. Growers planted and managed corn according to their normal practice. The surface foot of soil was sampled and analyzed for nitrate-and ammonium-N content before the grower applied mid-season sidedress N. Unfertilized check plots were left in each field. The checks were divided into three subplots, each four rows wide and 20 feet long. Three subplots were also picked randomly in the fertilized area adjacent to the checks. At harvest, corn from the center two rows of each subplot was harvested. The bottom eight-inch section of stalk was also cut from each of 10 plants in each subplot and analyzed for nitrate content.
We defined percent relative yield as the average yield from the unfertilized plots divided by the average yield from the fertilized plots. If yield from the unfertilized plots was less than 94 percent of yield from the fertilized plots, the site was identified as N responsive. The 94 percent cutoff allows for variability and considers the diminishing rate of return as increasing amounts of fertilizer are needed to attain ever smaller yield increases as maximum yield is approached. A 6 percent reduction represents a yield loss of about 0.5 tons/acre. Relative yield data were then compared to PSNT soil test data to determine if there is a soil nitrate concentration above which response to additional fertilizer N is unlikely. Relative yields were also compared to nitrate concentrations in corn stalks at harvest to determine if the test can be used to evaluate N sufficiency and N management efficiency.
On-farm PSNT Trial, 1996
Twelve growers participated in the on-farm PSNT trials. Plots were approximately one acre, or large enough to fill a truck at harvest. Growers limited pre-plant and at-planting N applications to a total of 50 pounds N/acre. A PSNT sample was collected and analyzed for nitrate-N before mid-season N applications,. Two top/sidedress N rates were then applied; 100 and 150 pounds N/acre. The goal of the project was to determine a PSNT soil test value above which a mid-season N application of 100 pounds N/acre would be sufficient for optimum yield. At three sites, treatments were replicated four times. Plots were harvested into separate trucks by growers. Yield information (weight and grade) was determined by the processor receiving the corn. Following harvest, soil and corn stalk samples from each N rate were analyzed for nitrate-N content. Nitrogen-responsive sites were defined as those where yield from the 100 pounds N/acre topdress was less than 98 percent of yield from the 150 pounds N/acre topdress.
Results
NWREC, 1995
Maximum sweet corn yield was attained at total N rate of 240 pounds N/acre. This yield, however, was not signficantly larger than with several combinations of N at planting and sidedressed N totalling as little as 120 pounds/acre (Table 6). At 120 pounds applied N/acre, yields tended to be reduced when all N was applied at planting. When 160 pounds N/acre was applied at planting, the PSNT value was 36 parts/million nitrate-N (Table 1) and additional sidedressed N did not improve yields or ear weight significantly (Table 6). All treatments including sidedress N at 80 lb/acre attained yields and ear weights not significantly different from the largest yield, regardless of the amount of N applied at planting. These results suggest that split N application may result in more efficient fertilizer use and is in agreement with past results obtained at NWREC.
SPAD meter readings were correlated with leaf N concentration and sweet corn yield (Tables 4, 6, and 7), suggesting that this meter may be useful for evaluating crop N status. The advantage of the SPAD meter is ease of use and instantaneous analysis. The meter has been used successfully in field corn production in many parts of the United States. Because of variability between sites, however, it may be necessary to establish a high N comparison plot in each field where the meter is to be used. The high N plot is used as a reference to determine if the rest of the field is N deficient.
Residual soil nitrate concentrations increased with both N at planting and sidedress N rates. Residual soil nitrate tended to be low at N rates at or below that needed for maximum yield (Table 8). As N rates exceeded crop demand, yields remained nearly constant while residual soil nitrate increased (Figure 1). Residual nitrate was about 35 and 80 pounds nitrate-N/acre for the 160 and 240 pounds N/acre rates, respectively, with no signficant increase in yield over this range of fertilizer rates. Timing of N application had no apparent effect on residual soil nitrate in the surface foot of soil.
Nitrate concentration in corn stalks at harvest has been used to evaluate crop N status for field corn. While a harvest test is obviously too late for correcting problems, the test can be useful for diagnosing causes of poor crop performance or evaluating efficiency of N management. Stalk nitrate concentrations at harvest remained low when N was not sufficient for maximum yield. As N rates increased beyond rates necessary for maximum yield, stalk nitrate concentrations also increased (Table 9). The data suggest that stalk nitrate concentrations above 2,000-4,000 ppm indicate that N applied was in excess of crop demand. More data are needed to better define a critical range.
NWREC, 1996
Highest yields of 10.7 tons/acre were obtained with a total N application of 120 pounds/acre (80 at planting, 40 mid-season; Table 10). Three other treatments gave yields not significantly different than the maximum. These were 120 pounds N (40 at planting and 80 mid-season), 160 pounds total N, and 200 pounds total N/acre. These results are consistent with those of 1995. Mean ear weight was highest at 160 pounds N/acre but several rates and combinations of at-planting and mid-season applications gave essentially equal ear sizes. When all 120 pounds N/acre were applied at planting, yield was significantly lower than when the same rate of N was split. Tipfill and ear length also increased with increasing rates of N. In these plots, the PSNT value was 26.4 parts/million when 120 pounds N/A was applied at planting. This rate of N at planting was not quite adequate to give maximum yields. However, when the same total rate of N was applied, but split into two applications, maximum yield was obtained. This is in agreement with past research indicating that splitting N applications may result in more efficient utilization of N by the corn plant.
SPAD readings and leaf N correlated with amount of N applied to the soil. However, these tests indicate only that the plant has sufficient N at the time of the test. These tests say nothing about whether the soil will provide sufficient N to maintain the plant through harvest. They may be of more use when the total N rate is divided into several applications such as with feeding liquid N through the irrigation system. However, the late SPAD readings (Tables 11, 12), taken three weeks after PSNT, correlated strongly with eventual yield. A SPAD reading of less than 45 at this point may indicate that additional N is needed.
Residual soil nitrate concentrations were lower than we usually experience in corn plantings (Table 11). Nevertheless, nitrate levels did increase with increasing rate of mid-season sidedressed N. About 68 pounds/acre residual nitrate- plus ammonium-N were present on plots fertilized at 200 pounds N/acre. This rate clearly exceeded that needed for maximum yield.
On-farm PSNT Trials, 1995
PSNT and yields are shown in Table 13 and plotted in Figure 2. The graph is divided into four quadrants. Quadrant III contains sites that had low PSNT values and were N responsive. Quadrant II contains sites that had high PSNT values and were not N responsive. Both of these quadrants contain "correct predictions," sites where the PSNT successfully predicted yield response to N fertilizer. Quadrant I contains sites that had low PSNT values but did not show yield response to added N. Quadrant IV contains sites that had high PSNT values and an unexpected yield response to additional N. These quadrants represent "incorrect predictions," sites where using PSNT would have resulted in the wrong decision.
The vertical line in Figure 2 represents the expected PSNT critical value of 25 parts/million nitrate-N, based on sweet corn research in New Jersey and field corn research in Oregon and several other states. The lack of high PSNT sites in this study prevented us from confirming the critical value for sweet corn in western Oregon.
PSNT values on 13 of the 16 sites were below 25 parts/million nitrate-N, suggesting that sidedress N applications are needed on most Willamette Valley sweet corn fields. Only site 16 had a PSNT value above 30 parts/million. This site had a history of manure applications and, therefore, the potential for large amounts of N mineralization.
Stalk nitrate data suggest a critical value of about 2,750 parts/million nitrate-N. If nitrate-N concentrations in the stalk at harvest are below 2,750 parts, lack of N may have limited yield. This is in agreement with the results from the NWREC plots.
Five sites (2, 5, 8, 10, 12) with low PSNT values did not show a difference in yield between fertilized and unfertilized plots (Figure 2, Quadrant I). At first, these appear to be incorrect predictions and raise questions concerning the effectiveness of PSNT. Closer examination of the data, however, shows that four of these sites had low yields in both the fertilized and unfertilized plots (Table 13). Corn stalk nitrate concentrations were also low in the fertilized plots at these sites, suggesting that the fertilized plots were still N deficient. Nitrogen deficiency on the fertilized plots may have prevented identification of a potential N response.
The small number of high PSNT sites suggests that determination of a critical value above which no sidedress N is needed may be of limited value. Nine of 16 sites, however, had PSNT values in the range of 17-25 ppm NO3-N. Sites in this range are close to the level is expected to be sufficient for maximum economic yield. Future research should focus on establishing a critical PSNT value above which sidedress N applications can be reduced, but not eliminated entirely.
On-farm PSNT Trial, 1996
PSNT values ranged from 8 to 31 ppm NO3-N (Table 14). The distribution of values in 1996 was similar to 1995 (Figure 3). Sites with PSNT values of 15 ppm or greater did not benefit from the higher N rate, with two exceptions (Figure 4). This indicates that if PSNT values are 15 ppm or greater, a mid-season N application of 100 pounds N/acre is sufficient for optimum yield. Because data is from a single year, a more conservative PSNT critical value of 18 ppm may be appropriate. The distribution of PSNT values indicates more than half of the sweet corn fields sampled during 1995-96 could benefit from this approach to N management.
The two N-responsive sites with high PSNT values were on coarse-textured soils in the Stayton and Coburg areas. They were also the only two sites where the low N plots were placed at the edge of the field. Whether the unexpected yield response was due to soil type, plot layout, or something else, is unknown.
The corn stalk nitrate test is designed to identify N deficiency or excess in the crop just harvested. This information would be used to adjust N management in future years. NWREC data in 1995 (above) suggested a stalk nitrate critical value of 2,750 ppm NO3-N. This critical value was accurate for 8 of 12 sites (Figure 5) in 1996. The two sites that were outliers for PSNT data were also outliers for corn stalk nitrate data (Tables 14 and 15).
Residual soil nitrate was higher on the high N plots than on the low N plots for 10 of 12 sites (Table 15). Figure 6 shows the relationship between residual soil nitrate and yield for the low N plots. Maximum yield was attained with the low N rate for all sites with residual soil nitrate above 50 pounds NO
3
-N/acre with two exceptions (same two outliers as for PSNT and stalk nitrate test). The data agree with previous estimates that residual soil nitrate above 50-75 pounds NO
3
-N/acre indicates possible opportunity for improved N management.
The coefficient of variation (CV) is a measurement of variability in data. On the replicated sites, the CV was extremely low (1.5 percent or less). The low CVs indicate that three replications are sufficient for future field-scale sweet corn research.
There were no statistically significant differences in corn grade (percent no value, grade, cobs per ton) on the replicated sites, as determined by the processors.
Summary
- Split N applications are more efficient than applying all N before or at planting for sweet corn. Maximum yield was obtained at N rates as low as 120 pounds N/acre with split applications at NWREC.
- The PSNT appears promising as an N management tool for sweet corn. Tentatively, PSNT values above 18 ppm nitrate-N indicate a mid-season N application of 100 pounds N/acre is sufficient for optimum yield. Confidence in the PSNT is greater on silty clay loams (Woodburn, Willamette, etc.) than on coarse textured and gravelly soils, such as those found in the Stayton and Coburg areas.
- The corn stalk nitrate at harvest test remains promising as an indicator of N management. Data from more years and sites are needed to establish a critical value.
- Residual soil nitrate can be reduced while maintaining optimum yield by using N management tools such as the PSNT. In most cases, reducing residual soil nitrate to 50 pounds nitrate-N/acre in the surface foot is a reasonable goal.
- Field-scale replicated trials resulted in good data that was believable to participating growers. For future trials, three replications are sufficient.
Table 3. Effect of N at planting on soil nitrate and ammonium concentration and sweet corn leaf N concentration five weeks after planting, NWREC, 1995 N at planting Soil nitrate-N Soil ammonium-N (lb/acre) (ppm) (ppm) 40 12.3 4.4 80 21.8 7.9 120 30.0 13.4 160 36.0 17.4 Significance ** * Table 4. Effect of N at planting and sidedressed N on dry weight, total N concentration, nitrate content, and SPAD readings of sweet corn leaves, NWREC, 7 July, 1995 Leaf N at planting Sidedress Dry wt. Total N N content SPAD (lb/A) N (lb/A) (g) (%) (g) reading 40 0 11.0 3.04 0.334 40.5 40 40 12.2 3.53 0.428 43.2 40 80 13.3 3.58 0.475 48.0 80 0 13.4 3.30 0.442 46.7 80 40 13.0 3.53 0.459 48.2 80 80 12.9 3.71 0.480 46.6 120 0 15.1 3.38 0.512 47.0 120 40 13.3 3.51 0.466 47.8 120 80 14.0 3.75 0.526 48.4 160 0 14.4 3.54 0.509 50.1 160 40 13.1 3.76 0.492 49.1 160 80 14.1 3.74 0.526 50.7 Significance N at planting NS * * ** Sidedress N NS ** NS ** Planting x sidedress NS NS NS ** Interaction LSD 2.6 Table 5. Effect of N at planting on soil nitrate and ammonium levels and leaf chlorophyll (SPAD) readings and N content at time of PSNT, NWREC, 1996 N at planting Soil nitrate-N Soil ammonium-N SPAD Leaf N (lb/acre) (ppm) (ppm) (%) 0 7.1 2.4 36.8 2.9 40 14.3 4.7 39.9 3.2 80 23.1 8.2 41.0 3.5 120 26.4 7.4 42.2 3.5 Significance ** ** ** * Table 6. Effect of N at planting and sidedressed N on yield and quality parameters of sweet corn, NWREC, 1995 N at Side- Ear Ear No. Ear Tipfillx plantingz dress Ny yield wt. ears per length (lb/A) (lb/A) (tons/A) (g) plot (inches) 40 0 4.3 216 42 8.3 2.6 40 40 6.3 235 56 8.8 3.0 40 80 9.3 306 63 9.1 3.8 80 0 7.5 266 55 8.6 3.4 80 40 9.1 287 66 8.9 3.5 80 80 9.2 301 64 9.2 3.7 120 0 8.6 276 65 8.7 3.6 120 40 8.7 293 62 8.8 3.6 120 80 9.7 297 68 9.0 3.4 160 0 9.3 311 63 9.0 3.8 160 40 9.3 304 64 9.3 3.5 160 80 10.6 309 71 8.9 3.9 Significancew N at planting ** ** * * N sidedress ** ** ** ** Plant x sidedress * * NS ** Interaction LSD 1.6 35 0.3 z40 lb/acre broadcast as 10-20-20 one day before planting. Remainder broadcast as urea one day after planting. yBroadcast as urea five weeks after planting. xFive-point scale with 5=perfect fill, 1=2 inches unfilled kernels. wNS,*,**: not significant and significant at 5% and 1% levels, respectively. Table 7. Correlation coefficients (Pearson's pairwise) for leaf dry weight, SPAD readings, leaf N concentration, and leaf N content, NWREC corn, 1995 Variable By variable Correlation coefficient Probability Leaf dry wt. N concentration 0.1927 0.367 SPAD N concentration 0.5981 0.002 SPAD Leaf dry wt. 0.3942 0.057 N content N concentration 0.6217 0.001 N content Leaf dry wt. 0.8866 0.000 N content SPAD 0.5836 0.003 Yield SPAD 0.8528 0.000 Mean ear wt. SPAD 0.7839 0.000 Table 8. Main effects of N at planting and sidedressed N on residual soil nitrate- and ammonium-N concentrations following sweet corn, NWREC, 1995 N at planting Nitrate Nitrate Ammonium Ammonium (lb/acre) 0-1 foot 1-2 foot 0-1 foot 1-2 foot -------------------ppm-N--------------------- 40 3.0 1.6 4.8 4.5 80 5.1 2.1 5.0 4.0 120 9.6 3.6 7.9 4.3 160 14.8 5.0 5.6 4.2 Significancez ** * NS NS Sidedressed N (lb/acre) 0 3.9 1.6 4.4 3.9 40 7.3 3.2 6.4 4.3 80 13.2 4.4 6.8 4.1 Significance ** * NS NS zNS,*,**: not significant and significant at 5% and 1% levels, respectively. Table 9. Effect of total N applied on corn yield and stalk nitrate-N content, NWREC, 1995 Total N Applied, lb/A Yield, tons/A Stalk nitrate-N, ppm 40 4.3 30 80 6.9 271 120 9.0 1107 160 9.1 4310 200 9.5 7146 240 10.6 9203 LSD (0.05) 1.6 1319 Table 10. Effect of N at planting and mid-season sidedressed N on yield and quality parameters of sweet corn, NWREC, 1996 N at Mid-season Yield No. Ear Ear Tipfillz planting N ears/ wt. length (lb/A) (lb/acre) (tons/A) plot (g) (inches) 0 0 2.7 55 102 7.1 1.5 0 40 7.0 81 181 7.7 2.1 0 80 8.7 83 218 8.1 2.5 40 0 6.3 78 169 8.2 2.2 40 40 8.1 86 196 8.2 2.2 40 80 10.1 90 236 8.3 2.5 80 0 8.5 87 203 7.9 1.9 80 40 10.7 94 238 8.1 2.3 80 80 9.1 86 225 8.3 2.4 120 0 9.0 87 217 8.3 2.7 120 40 10.2 89 240 8.4 2.6 120 80 9.4 90 218 8.4 2.7 LSD (0.05) 1.6 10 28 0.6 zBased on 5-point scale with 5 = perfect fill, 1= 2 inches unfilled kernels. Table 11. Effect of N at planting and mid-season N on leaf chlorophyll (SPAD) readings three weeks after second N application and residual soil nitrate and ammonium content, NWREC, 1996 N at planting Mid-season N Late SPAD Soil nitrate Soil ammonium (lb/acre) (lb/acre) (ppm) (ppm) 0 0 31.3 1.8 4.9 0 40 40.5 1.6 4.2 0 80 46.2 5.0 5.1 40 0 41.3 1.4 4.3 40 40 45.6 2.7 4.2 40 80 48.2 5.4 4.7 80 0 46.6 2.5 5.7 80 40 47.6 5.0 5.6 80 80 46.2 4.0 5.2 120 0 46.4 3.3 7.1 120 40 49.0 2.6 7.2 120 80 50.3 10.3 6.7 LSD (0.05) 3.9 3.1 0.7 Table 12. Correlation coefficients (Pearson's pairwise) for SPAD readings, leaf N concentration, and sweet corn yield, NWREC, 1996 Variable By variable Correlation coefficient Probability PSNT-SPAD Leaf N 0.7649 0.0007 PSNT-SPAD Yield 0.4592 0.0006 PSNT-SPAD Tipfill 0.1534 0.2777 PSNT-SPAD Ear wt. 0.3980 0.0035 Late-SPAD Yield 0.8818 0.0000 Late-SPAD Tipfill 0.6477 0.0000 Late-SPAD Ear wt. 0.8863 0.0000 Table 13. Sweet corn yield and stalk nitrate concentrations at harvest, grower cooperator sites, 1995 Site PSNTz Stalk nitrate Yield Relative (ppm) (ppm) (T/A) yield Nitrate-N No added N Added N No added N Added N (%) 1 11 42 4156 7.7 9.9 78.2 2 11 498 756 7.8 7.4 104.8 3 12 2262 7261 9.1 9.7 93.4 4 12 1925 2806 10.1 11.6 87.2 5 17 2297 814 8.4 8.0 105.1 6 17 657 3943 12.2 13.1 93.0 7 17 120 3219 8.1 9.9 82.3 8 19 892 1871 9.5 9.3 101.8 9 20 1248 3376 9.4 10.3 91.2 10 22 2379 4826 11.7 10.9 107.8 11 22 1473 3242 9.5 13.7 69.3 12 23 893 2312 9.3 9.4 98.6 13 24 447 5793 9.9 11.1 89.0 14 29 6224 10573 8.7 9.5 91.2 15 29 4489 6290 9.1 9.2 98.8 16 47 9044 10309 10.4 11.2 93.5 zSoil nitrate-N concentration at time of sidedressing. Table 14. Sweet corn yield at two N ratesz, grower-cooperator sites, 1996 Site PSNT, ppm Yield (T/A) Relative yield Nitrate-N Low N High N (%) 1 8 9.4 10.1 93.5 2 11 9.7 10.1 95.7 3 12 12.5 12.5 100.0 4 15 10.2 10.4 98.5 5 18 9.7 10.6 91.3 6 21 9.7 9.8 99.0 7 21 8.7 8.9 98.0 8 22 10.7 10.9 98.7 9 24 9.6 8.7 110.6 10 26 393 385y 102.2 11 26 12.4 12.1 102.4 12 31 8.2 9.0 92.2 zLow N rate = 100 lb/A after PSNT. High N = 150 lb/A after PSNT. yFresh market grower. Yield in cases/A. Table 15. Residual soil nitrate and cornstalk nitrate at two Nz rates, grower-cooperator sites, 1996 Site Residual soil Nitrate-N (lb/A) Cornstalk Nitrate-N (ppm) Low N High N Low N High N 1 12 20 2219 5761 2 16 26 5358 6124 3 91 113 8131 7318 4 43 37 9219 8860 5 110 296 3584 2918 6 75 123 9385 9089 7 17 21 9586 10575 8 -- -- 8364 11288 9 96 149 6975 7950 10 142 242 6286 5461 11 92 107 2467 3630 12 64 46 9492 7013