Introduction
Vegetable growers in the Willamette Valley use high rates of N fertilizers, often exceeding 250 to 300 pounds actual N/acre per 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 significantly 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 three-year survey of residual mineral N in grower fields indicates that residual N tends to be greatest following sweet corn. 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 significant 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 inputs of manures 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 preliminary evaluations of the PSNT in 1995 and 1996 (Hemphill, 1997) convinced us that the potential exists to reduce sidedress N applications to sweet corn significantly in certain circumstances, resulting in reduced cost to growers and reduction in leachable residual nitrate. The results reported here continued this effort and resulted in a modification of the PSNT, which appears applicable to sweet corn production on typical Willamette Valley soils.
Methods, NWREC Experiment, 1997
'Jubilee' sweet corn was seeded on 15 May on 30-inch row spacing. The plots had previously been fallow for two years. Nitrogen rates of 0, 50, 100, 150, and 200 pounds/acre were applied at planting to establish five different levels of early-season soil nitrate (Table 2). Soil samples for PSNT were taken on 7 July and sidedress N rates of 0, 30, 60, 90, and 150 pounds/A were applied to the appropriate plots on 10 July. All N was applied as urea. Table 1 contains the complete list of treatments. One treatment (13) received 50 pounds N/acre at planting, followed by 25-pound applications on 10, 18, and 29 July, and 12 August, for a total N application of 150 pounds/acre. SPAD-502 chlorophyll meter (Minolta Co., Ltd., Japan) readings were taken on intact leaves on 11, 18, and 29 July, and 12 and 21 August. Harvest was on 20 August. Plots were sampled to 1-foot depth for residual soil nitrate on 21 August.
Methods, NWREC Experiment, 1998
'Jubilee' sweet corn was seeded on 9 June on 30-inch rows. The plots had previously been fallow for two years. Nitrogen rates of 0, 40, 80, 120, and 180 pounds/acre were applied at planting to establish five different levels of early-season soil nitrate (Table 7). Soil samples for PSNT were taken on 21 July. Sidedress N rates of 0, 20, 60, 80, 100, 140, and 200 pounds/A were applied to the appropriate plots on 22 and 29 July, and 14 August, to complete 11 different combinations of N rate, form of N applied, and timing of N application (Table 2). All N was applied as urea except for applications of Greenfeed (TM) 27-0-0 liquid controlled release N solution to corn foliage on Treatments 15 and 16 (Table 3). The Greenfeed material contains 7.1% urea plus 19.4% urea derivatives (polymethylene urea, monomethylol urea, methylene diurea) which break down relatively slowly. SPAD meter readings were taken on 10 intact leaves/plot at approximately weekly intervals starting on 16 July and continuing at approximately weekly intervals through harvest. Harvest was on 14 September. Plots were sampled for residual soil nitrate on 18 September. A second variety, GH 1703 (Rogers Sandoz), was also seeded on 9 June and treated with only 5 rates of N (Table 3). The purpose was to compare the performance of the SPAD meter over two varieties.
Methods, On-farm Trial of the Pre-sidedress Soil Nitrate Test (PSNT), 1997 and 1998
As in 1996 (Hemphill, 1997), the PSNT was evaluated in 1997 and 1998 at nine grower locations in which plots of approximately 1 acre were compared to the grower practice in the remainder of the field for ear yield, stalk nitrate-N at harvest, and soil nitrate-N at harvest. The assumption of the PSNT is that soil can be tested at about the 1-foot stage of corn growth (about 5 weeks after planting) and that sidedress fertilizer application can be reduced when soil nitrate content exceeds a critical level. The soil was sampled to a depth of 12 inches and analyzed for nitrate-N content. Sufficient corn was harvested from the plot area to fill a truck. The harvested area was measured, the contents of the truck weighed, and the yield calculated. A similar procedure was followed to provide a matching sample from an adjacent portion of the field in which the standard grower practice was followed. At harvest, 12-inch segments of corn stalks from just above the brace roots were cut, dried, ground, and analyzed for nitrate-N concentration. A soil sample was taken at the same time and also analyzed for nitrate-N content.
In 1997, growers limited pre-plant and at-planting N application to 50 pounds/acre. Treatments were the grower's standard sidedress N rate and one-half this rate. Three of the sites were replicated. Sites considered responsive to N were those where yield from the half-rate of sidedress N was less than 98% of the yield from the full rate of sidedress N. A relative yield of 98% was chosen as the yield below which the cost of additional N fertilizer would be recovered in additional yield. For the calculation, we assumed a price of $85/ton for corn, a 10 ton/acre yield, and a cost for N of $0.32/lb.
In 1998, our goal was to have 175 pounds available N/acre provided from a combination of the soil and sidedress N application. The soil samples were analyzed for nitrate-N and an approximation of the amount of nitrate-N in the soil (pounds/acre) was made using the conversion parts/million multiplied by 3.6 = pounds/acre in the surface foot of soil. Sufficient sidedress N was applied to provide a total of 175 pounds N/acre. The fertilizer N was applied by the growers. A relative yield level of 97.5% was chosen based on the current prices for urea fertilizer ($195/ton), the value of sweet corn at $85/ton, and fertilizer application cost of $5/acre. At this relative yield, a yield increase of 0.25 tons/acre would just offset the cost of an additional 75 pounds N/acre. Thus, a relative yield level of 97.5% is a reasonable measure of success of the PSNT prediction for needed N with 1998 prices.
Results, NWREC, 1997
Our past trials indicated that the critical level of PSNT is about 25 ppm; that is, a positive yield-response to additional N is not to be expected when the soil nitrate concentration is at this level 4 to 6 weeks after planting. However, in this experiment, application of 100 pounds N/acre at planting resulted in a PSNT of 47 ppm (Table 2) but this level of N was not adequate for maximum yield (Table 4). This is probably because achieving this level of available N as a result of fertilizer application does not indicate a continuing reservoir of available N coming from mineralization of organic matter. In contrast, in the grower trials, much lower levels of N were usually applied at planting, resulting in a better estimate of nitrate-N availability from the soil.
The highest yield was obtained with 200 pounds N/acre, with all the N applied at planting. Yields tended to decline at higher N rates. Past trials have indicated greater yield with split N application. In this trial a breakdown of fertilizer banding equipment prevented banding N below the seedline. A broadcast application of the initial 50 lb N/acre (Treatment 14) may not have provided sufficient N during early growth. In general, our experiments have indicated that, while it is advisable to delay the application of the bulk of crop N needs until after rainfall has tapered off, it is critical to have adequate N present at all times. This can be seen clearly in the case of Treatment 13 (Table 4), for which yield was much lower than other treatments with similar N rates (Trts. 4, 5, 6, 12). Apparently, delaying the application of much of the N until relatively late in crop development meant N deficiencies at critical earlier stages of growth.
Sweet corn yield is plotted vs. total N rate in Figure 1. Most yields clustered between 8 and 9 tons/acre, so a smooth curve cannot be fitted to the data. However, three points stand out: the high yields at 200 lb N/acre and the low yield of 7.6 tons/acre for Trt. 13. SPAD readings also tend to cluster (Table 5 and Figure 2) but the relationship between SPAD and N applied can be seen in Figure 2. SPAD reading at harvest increased linearly with rate of N applied up to about 180 pounds N/acre. After this, the SPAD reading tends to plateau and not be responsive to more N. There is no good explanation for the one obvious outlier at 200 lb applied N. A similar relationship existed for earlier SPAD readings.
Figure 3 demonstrates how SPAD readings tend to vary during the season. Typically, the readings start low, increase, and then gradually decline toward harvest. The middle curve in the figure is atypical. This is the curve for the 150 lb treatment (Trt. 13) in which the N was applied in six separate applications of 25 or 50 lb/acre. Dribbling on the fertilizer in this fashion resulted in very steady SPAD readings but, as seen above, yield suffered. Figure 4 indicates that there is not a simple relationship between SPAD at harvest and yield. Points are clustered between SPAD readings of 37 and 44. This is because of the great variation in this experiment among different N rates at planting and at harvest. The timing of the N application as well as the total amount applied affects the SPAD reading. For SPAD to be an effective tool in sweet corn fertilization, we will need to generate more data of the type in Figure 3 and standardize the method and timing of N application.
Both soil nitrate and ammonium concentrations were elevated at time of harvest, but only for high rates of applied N (Table 6). The high levels of residual N at 150 pounds applied N/acre were likely due to half the total N being applied to this treatment (#13) after the normal sidedress time. Almost 80 pounds nitrate-N/acre were left behind in the surface foot of soil by the corn crop fertilized with 200 pounds N/acre.
Results, NWREC, 1998
As noted above, preliminary trials in 1995 and 1996 indicated that the critical level of PSNT is about 25 ppm. In this experiment, application of 120 pounds N/acre at planting resulted in a PSNT of 29.6 ppm (Table 7). A treatment which provided a total N application of 120 lb/acre (80 at planting, 40 sidedress at mid-season) produced maximum yield (Tables 8 and 9), indicating that a PSNT level of 25?30 ppm might have produced maximum yield, but this was not tested directly.
The highest yield of both 'Jubilee' and 'GH 1703' was obtained with 120 pounds N/acre, with 80 pounds/acre applied at planting and the remainder 6 weeks later. Yields tended to decline at higher N rates. Past trials have indicated greater yield with split N application, but with maximum yield at 180-200 lb/acre. One explanation for the highest yields at a lower-than-expected N rate may be seen in Table 7. Available N in the soil before planting was unusually high, nearly 50 pounds N/acre as nitrate and 15 as ammonium. If this amount of available N already present in the soil is added to the 120 pounds/acre applied N, then the highest yield was obtained at 185 pounds available N/acre, very much consistent with previous trials. This serves to reinforce the importance of the PSNT, as high concentrations of available N are occasionally present in Willamette Valley soils, even after a high-rainfall winter like that of 1997?1998.
Averaged over all N rates, 'GH 1703' outyielded 'Jubilee' by 0.9 tons/acre (Table 8). However, the difference in yield was greater at the suboptimal rates of N (Table 9). Mean ear weight and tipfill responded less to increasing N rate for 'GH 1703' than for 'Jubilee'. The two varieties also had a strikingly different appearance in the field. Leaves of 'GH 1703' were darker green and thicker, and the color remained dark green until harvest even at the zero rate of N. If confirmed in future testing, the ability of a variety to produce acceptable yields at lower rates of applied N may be of great help in avoiding leaching of nitrate and in reducing fertilizer costs.
Treatments 11 through 14 were added to compare the performance of 'Jubilee' at 180 pounds/acre applied N, but with varying rates applied at planting versus mid-season and later sidedressings. Yields of these treatments are seen in rows 4?7 of Table 10. None of these treatments produced a higher yield or quality than the standard method of 40 pounds N/acre at planting and 140 pounds/acre sidedressed 6 weeks after planting (Treatment 7, 10.1 tons/acre, line 7 in Table 9). While applying relatively small amounts of N at several times during the season could affect residual soil N or N leaching, it is clear from this and previous work at NWREC that there is no yield advantage to delaying N applications past mid-season. However, note from Table 11 that treatments 11, 12, and 13 (lines 6, 7, and 8) resulted in lower residual soil N than did applying all the sidedress N at one time (line 4).
In treatments 15 and 16, we compared corn yield with all or a portion of the sidedress N applied as Greenfeed 27-0-0 controlled-release N at 60 and 120 pounds N/acre, respectively, with the corresponding treatments in which all sidedress was applied as urea on 22 July (compare Table 10, lines 2 and 3 with Table 9, lines 3 and 5). The use of Greenfeed 27-0-0 did not provide a yield advantage compared to the use of urea alone as N source.
SPAD readings started off higher than last year, reflecting the higher level of nitrate in the soil at planting. At time of sidedress N application, SPAD readings did not vary with application of either 0 or 40 lb N/acre at planting (Table 12). Differences among total N rates were not evident until two weeks later. SPAD readings for treatments receiving suboptimal N started to decline 3 to 4 weeks before harvest. The thicker, darker green leaves of 'GH 1703' gave consistently higher SPAD readings than did 'Jubilee', indicating that separate calibration curves will be needed for each variety. As in 1997, applying small amounts of sidedress N four times during the growing season tended to keep SPAD readings more level over time (Table 13) but, as noted above, did not result in improved yield.
As in 1997, both soil nitrate and ammonium concentrations were elevated at time of harvest but only for high rates of applied N of 180 pounds/acre or more (Table 11). The residual nitrate-N level of about 40 pounds/acre for the optimal N rate of 120 pounds/acre is probably acceptable and certainly is no higher than at planting. However, at 180 and 240 pounds N/acre, residual nitrate-N varied from 60 to 160 pounds/acre, depending on N rate and timing of application. Residual nitrate levels were generally higher in 1998 than in 1997 for all but the lowest rates of applied N. Residual ammonium levels did not vary significantly with amount of N applied.
Results, On-farm PSNT Trial, 1997
PSNT values ranged from 8.8 to 38.8 ppm nitrate-N, similar to those found the previous two years (Table 14). The conclusion from 1996 was that 100 pounds sidedress N/acre was sufficient for at least 98% relative yield when the PSNT value was 18 ppm or more. Our approach in 1997 was slightly different as the sidedress N rates varied by grower rather than being set at 100 vs. 150 pounds/acre. In addition to this variation in experimental procedure, the analysis of the data was modified.
Relative yield was plotted vs. the sum of soil nitrate-N (as determined by PSNT) and sidedressed fertilizer N (Fig. 5). The horizontal line in the Cate-Nelson type graph represents 98 percent relative yield. The goal of the Cate- Nelson technique is to manipulate the vertical and horizontal lines such that the greatest number of points is located in the upper right (I) and lower left (IV) quadrants of the graph. Points in these two quadrants are viewed as "correct" predictions. The lower left quadrant (IV) represents situations in which the PSNT would have called for additional N and prediction was correct. The points in the upper right quadrant (I) represent situations in which the test indicated that available N was adequate for 98 percent relative yield and the prediction was correct.
Points falling in the upper left and lower right quadrants are considered "incorrect" predictions. The upper left quadrant (III) represents situations in which the PSNT called for additional N, but the relative yield indicates that it was not needed. These incorrect predictions are of low risk to the grower as they call for an additional fertilizer application that the grower would likely make anyway unless that grower typically uses rates that are considerably less than most growers now apply. Yield would not be sacrificed and expenditures would be the same save for the cost of the PSNT itself. However, these situations are potentially the most damaging to the environment because excess N at the end of the growing season will be leached by the heavy winter rainfall typical of western Oregon.
Points falling in the lower right quadrant (II) are situations where the plots did not achieve a relative yield of 98 percent when the PSNT indicated that N was adequate. These sites were on sandy or gravelly soils in which the reservoir of mineralizable soil N was very low.
Yield data from both 1996 and 1997 is presented in Figure 6. The graph reveals that 98 percent or greater relative yield was attained when the sum of the PSNT and sidedress N was greater than 175 pounds/acre. If the sum of PSNT and sidedress N is less than 150 pounds/acre the relative yield is less than 98 percent. Between 150 and 175 pounds/acre, the relative yield is sometimes less than 98 percent and sometimes greater than 98 percent.
Assuming that the sum of PSNT plus sidedress N must be 175 pounds/acre to achieve the desired relative yield of 98 percent, the amount of sidedress N to apply can be calculated from the formula:
Sidedress N (pounds/acre) = 175 pounds/acre - [PSNT (ppm) x 3.6] (1)
Another method to provide sidedress N rates is to group PSNT results into three categories as shown in Table 15.
Stalk nitrate concentrations at harvest (Table 14) did not provide an adequate assessment of yield or N treatment. The critical value of 2700 ppm that we proposed in 1996 (Hemphill, 1997) seems very low for 1997 data. In addition, a sufficient nitrate-N concentration of 10,000 ppm in stalks at harvest was found in New Jersey (Heckman and Prostak, 1992). Our current data set shows that high stalk nitrate-N content, probably above 8,000 ppm, indicated adequate N supply, but that stalk concentrations below 8,000 ppm did not necessarily indicate insufficient N supply.
Results, On-farm PSNT Trial, 1998
The 1998 trial was a test of the formula (1) proposed above. Yields ranged from a low of 7.2 tons/acre at site G, which had an infestation of barnyard grass, to a high of nearly 14 tons/acre at site B (Table 16). To compare yield from all sites, relative yield was calculated by dividing the yield of the PSNT plot by the yield from the sample of the rest of the field and multiplying by 100 percent. For example, if yield from the field was 10.0 tons/acre and that from the PSNT plot was 9.75 tons/acre, the relative yield is 97.5 percent.
Relative yield was lowest (92.3 percent) at site E and highest (103.8 percent) at site B. Relative yield is ranked against the sum of sidedress N fertilizer and the soil nitrate-N level determined by the PSNT (Table 17). The lowest amount of N, 133 pounds/acre, was entirely soil N. No N fertilizer was sidedressed at this site. The plot yield was almost 96 percent of the yield from the rest of the field which had a sidedress N application of 160 lb/acre. Four of the plots received less than 175 pounds N/acre as a combination of soil N and sidedress N. Of these four sites, only one (A) produced a relative yield greater than 97.5 percent and that site had a combined soil and applied N level of 170 pounds/acre.
Of the five sites with more than 175 pounds N/acre from soil and sidedress, two produced relative yields less than 97.5 percent. Site H was a sandy soil; at harvest soil and stalk nitrate-N content at site H were lower for the plot area than for the rest of the field. Soil and stalk nitrate-N content at harvest were both low for site D. The stalk NO3-N concentration was lower in the plot area than in the field (Table 16).
Relative yields for the last three years were plotted versus the sum of sidedress and soil N (Figure 7). The vertical line on the right of Figure 7 represents 175 pounds/acre; the left vertical line marks 135 pounds N/acre. This line creates a third category of data points where additional N produces additional yield about half the time i.e., between 135 and 175 pounds N/acre there are five points above, and five points below, the 97.5 percent relative yield line. Another way to express the test outcome is "yes, maybe, no." If sidedress plus soil N is below 135 pounds/acre, then, "yes" ?a yield increase from additional N is highly likely. If the soil plus sidedress N is above 175 pounds/acre, then, "no" ?increased yield with additional N in highly unlikely except on sandy or gravelly soils. If the N level is between 135 and 175, then, "maybe" ?a yield increase with more N is difficult to predict. For fields in the "maybe" category, the SPAD meter may be useful in monitoring plant N status.
For the last three years, nitrate-N content of the soil at time of PSNT ranged from 30 to 160 pounds/acre in the surface foot. This range of available N shows the need for a test that enables growers to adjust sidedress N application rates on a site-specific basis. Our modification of the PSNT allows many growers to make this adjustment. Of the 30 locations reported here, a detrimental economic consequence of using the modified PSNT occurred in only two locations.
Literature Cited
Heckman, J.R. and D. Prostak. 1992. Presidedress soil nitrate test (PSNT) recommendations for sweet corn. Rutgers Coop. Ext. and N.J. Agric. Expt. Sta. FS 760.
Hemphill, D.D., Jr. 1997. Vegetable research at the North Willamette Research and Extension Center, 1995?1996. Oregon Agric. Expt. Sta. Spec. Rep. No. 975.
Marx, E. 1995. Evaluation of soil and plant analyses as components of a nitrogen monitoring program for silage corn. M.S. Thesis, Oregon State Univ., Corvallis.
Table 1. List of N rates (lb/acre), PSNT trial, NWREC, 1997. Trt. Total N rate At planting Sidedress, 7/10 1 0 0 None 2 80 50 30 3 110 50 60 4 140 50 90 5 130 100 30 6 160 100 60 7 190 100 90 8 180 150 30 9 210 150 60 10 240 150 90 11 200 200 0 12 150 150 0 13 150 50 4x25 14 200 50 150 Table 2. Effect of N at planting on soil nitrate levels and leaf SPAD readings at time of PSNT testing, NWREC, 1997. N at planting, lb/acre Soil nitrate, ppm, 7/7 SPAD, 7/11 0 3.7 37.3 50 20.3 42.0 100 47.0 43.8 150 56.4 44.9 200 71.3 44.9 LSD (0.05) 35.5 4.2 Table 3. List of treatments in PSNT sweet corn experiment, NWREC, 1998. Trt. Variety Total N N at Sidedress N applied planting 7/22 7/29 8/12 ----------------lb/acre------------------- 1 Jubilee 0 0 0 0 0 2 GH 1703 0 0 0 0 0 3 Jubilee 60 40 20 0 0 4 GH 1703 60 40 20 0 0 5 Jubilee 120 40 80 0 0 6 GH 1703 120 40 80 0 0 7 Jubilee 180 40 140 0 0 8 GH 1703 180 40 140 0 0 9 Jubilee 240 40 200 0 0 10 GH 1703 240 40 200 0 0 11 Jubilee 180 40 60 40 40 12 Jubilee 180 80 40 40 20 13 Jubilee 180 120 20 20 20 14 Jubilee 180 180 0 0 0 15 Jubilee 60 40 10z 0 10z 16 Jubilee 120 40 40y 20z 20z __________________________________________________________ zApplied as Greenfeed 27-0-0 controlled release N solution. yApplied as one-half urea, one-half Greenfeed 27-0-0. Table 4. Yield of sweet corn, PSNT plots, NWREC, 1997. Trt. Total N Yield Ear length Ear wt. Tipfill (lb/acre) (tons/acre) (inches) g/ear 1 0 3.4 8.4 249 2.3z 2 80 8.0 9.1 321 3.1 3 110 8.5 9.7 320 3.1 5 130 8.1 9.6 316 2.9 4 140 8.6 9.8 317 3.0 12 150 8.8 9.7 324 3.0 13 150 7.6 9.5 296 2.8 6 160 8.2 9.8 330 3.1 8 180 8.7 9.7 319 3.1 7 190 8.5 9.5 332 3.2 11 200 10.1 9.5 321 3.2 14 200 9.5 9.4 283 2.9 9 210 8.9 9.6 349 3.1 10 240 8.9 9.2 314 2.9 LSD (0.05) 1.6 0.5 28 0.6 zFive-point scale with 5 = perfect fill, 1 = 2 inches or more unfilled kernels. Table 5. Trends in SPAD readings as affected by total N application, NWREC, 1997. Trt. Total N, lb/acre SPAD, 7/18 SPAD, 7/29 SPAD, 8/12 SPAD, 8/21 1 0 38.5 29.5 22.9 19.5 2 80 48.7 50.8 40.9 34.7 3 110 47.3 47.7 39.7 38.0 5 130 49.8 52.4 43.9 38.9 4 140 48.1 48.8 43.3 40.3 12 150 50.8 50.0 41.8 37.7 6 160 50.3 51.8 44.4 42.5 8 180 50.2 52.5 46.3 44.5 7 190 52.0 49.8 45.2 43.4 11, 14 200 49.7 51.5 44.7 41.3 9 210 51.0 53.2 42.6 43.4 10 240 50.4 53.2 45.0 42.3 13 150z 44.0 (75) 43.4 (100) 43.0 (125) 42.7 (150) LSD (0.05) 2.8 5.2 5.9 4.7 zTotal at end of season. Actual amounts applied before each SPAD measurement are in parentheses after the SPAD mean. Table 6. Effect of total N rate on residual soil N levels in the surface foot of soil, NWREC, 1997. Total N, lb/acre Residual nitrate, ppm Residual ammonium, ppm 0 0.9 4.6 80 2.7 6.0 110 2.9 5.2 140 4.0 6.3 150z 14.9 14.3 200 19.9 13.3 LSD (0.05) 5.2 NS zThis treatment (#13) had a large proportion of the total N applied relatively late in the season. Table 7. Effect of N applied at planting on soil nitrate and ammonium levels at time of sidedress N application, NWREC, 1998. N rate, lb/acre Soil nitrate-N, ppm Soil ammonium-N, ppm 0 13.1 3.8 40 22.3 4.1 80 23.8 5.1 120 29.6 6.4 180 31.1 9.5 Significance *** *** ***Significant at 0.1% level. Table 8. Main effects of rate of applied N and cultivar on yield of sweet corn, NWREC, 1998. Yield No. of Mean ear Tipfillz Ear length tons/acre ears/plot wt. (g) inches N rate, lb/A 0 7.3 55 210 2.2 7.7 60 9.2 58 251 2.6 8.3 120 10.9 62 277 2.9 8.6 180 10.4 59 275 2.8 8.5 240 10.4 59 275 2.7 8.6 Significance *** * *** * ** Cultivar Jubilee 9.2 59 244 2.7 8.4 GH 1703 10.1 58 272 2.5 8.2 Significance ** NS *** NS NS z5-point scale with 5 = perfect fill, 1 = 2 or more inches unfilled. ***,**,*,NSSignificant at p=0.1, 1, and 5% levels, and non-significant, respectively. Table 9. Interaction of rate of applied N and cultivar on yield of sweet corn, NWREC, 1988. N rate Cultivar Yield No. of Mean ear Tipfillz Ear length lb/acre tons/acre ears/plot wt. (g) inches 0 Jubilee 6.3 55 179 1.8 7.6 GH 1703 8.4 54 241 2.5 7.8 60 Jubilee 8.7 58 233 2.6 8.4 GH 1703 9.7 57 268 2.5 8.2 120 Jubilee 10.7 63 265 3.1 8.8 GH 1703 11.2 60 290 2.7 8.3 180 Jubilee 10.1 59 266 3.2 8.9 GH 1703 10.8 59 284 2.3 8.0 240 Jubilee 10.4 59 276 3.0 8.6 GH 1703 10.4 59 275 2.5 8.6 LSD (0.05) 0.6 5 30 0.1 z5-point scale with 5 = perfect fill, 1 = 2 or more inches unfilled. Table 10. Effects of total rate of applied N, proportion of N applied at planting, split applications of N, and N source on yield of 'Jubilee' sweet corn, NWREC, 1998. Total N applied N at planting Yield No. of Mean ear Tipfillz Ear length lb/acre lb/acre tons/acre ears/plot wt. (g) inches 0 0 6.3 55 179 1.8 7.6 60 40y 8.9 59 237 2.7 8.3 120 40x 9.7 62 246 2.4 8.1 180 40w 9.8 61 251 2.9 8.4 180 80v 10.0 61 257 2.9 8.4 180 120u 9.5 61 244 2.7 8.5 180 180 9.5 61 245 2.8 8.5 LSD (0.05) 1.0 5 26 0.1 z5-point scale with 5 = perfect fill, 1 = 2 or more inches unfilled. ySidedress N source: Greenfeed 27-0-0, 10 lb/acre, each, on 22 July and 12 August. xSidedress N source: Greenfeed 27-0-0 and urea. 20 lb/acre each of Greenfeed and urea applied on 22 July, 20 lb/acre as Greenfeed on 29 July and 12 August. wSidedress N source: urea, applied at 60 lb/acre on 22 July, 40 lb/acre on 29 July, and 40 lb/acre on 12 August. vSidedress N source: urea, applied at 40 lb/acre on 22 July, 40 lb/acre on 29 July, and 20 lb/acre on 12 August. uSidedress N source: urea, applied at 20 lb/acre, each, on 22 July, 29 July, 12 August. Table 11. Effects of total rate of applied N, proportion of N applied at planting, and split applications of N on post-harvest soil nitrate and ammonium concentrations, NWREC, 1998. N rate, lb/acre Soil nitrate-N, ppm Soil ammonium-N, ppm 0 2.6 4.8 60 2.9 3.5 120 11.8 6.3 180 37.6 5.7 240 40.8 9.4 180z 20.5 7.9 180y 16.7 6.4 180x 21.9 9.3 LSD (0.05) 13.5 NS zSidedress N source: urea, applied at 60 lb/acre on 22 July, 40 lb/acre on 29 July and 40 lb/acre on 12 August. ySidedress N source: urea, applied at 40 lb/acre on 22 July, 40 lb/acre on 29 July, and 20 lb/acre on 12 August. xSidedress N source: urea, applied at 20 lb/acre, each, on 22 July, 29 July, 12 August. Table 12. Main effects of rate of applied N and cultivar on SPAD chlorophyll measurements in sweet corn, NWREC, 1988. Date of SPAD measurement 16 July 22 July 27 July 3 Aug. 10 Aug. 17 Aug. 24 Aug. 1 Sept. 15 Sept. N rate, lb/acre 0 42.1 40.8 46.8 45.5 44.0 40.1 37.9 36.1 29.0 60 42.6 41.5 50.7 48.9 49.2 44.1 45.8 41.8 34.5 120 43.0 43.4 50.1 51.1 52.5 48.1 50.0 48.6 44.4 180 42.8 42.5 51.5 51.5 50.7 49.8 50.9 48.6 45.3 240 42.0 41.4 51.8 51.7 50.2 48.2 50.5 48.7 43.7 Significance NS NS *** *** *** *** *** *** *** Cultivar Jubilee 40.0 38.5 46.3 45.8 44.1 41.4 41.8 39.1 33.1 GH 1703 44.9 45.3 54.0 53.7 54.5 50.7 52.2 50.4 45.6 Significance *** *** *** *** *** *** *** *** *** ***,NSSignificant at 0.1% level and nonsignificant, respectively. Table 13. Effects of total rate of applied N, proportion of N applied at planting, split applications of N, and N source on SPAD chlorophyll measurements in 'Jubilee' sweet corn, NWREC, 1998. Total N N at planting Date of SPAD measurement applied, lb/acre lb/acre 16 July 22 July 27 July 3 Aug. 10 Aug. 17 Aug. 24 Aug. 1 Sep. 15 Sep. 0 0 39.9 37.4 42.5 41.7 37.8 33.9 29.9 29.2 20.4 60 40z 40.9 39.8 46.7 45.7 43.9 40.0 40.7 35.3 31.0 120 40y 38.5 37.7 46.1 45.2 43.9 42.6 44.6 41.7 37.7 180 40x 40.3 38.7 45.5 45.9 42.2 42.3 43.9 42.7 38.3 180 80w 39.2 39.5 46.9 45.6 42.5 41.6 43.7 42.8 38.4 180 120v 39.8 40.5 48.3 47.7 44.3 41.5 41.9 41.5 37.5 180 180 37.6 41.1 48.6 46.6 43.8 42.0 42.8 40.5 36.8 LSD (0.05) NS 2.9 2.6 3.0 4.1 4.6 5.3 4.7 4.1 zSidedress N source: Greenfeed 27-0-0, 10 lb/acre, each, on 22 July and 12 August. ySidedress N source: Greenfeed 27-0-0 and urea. 20 lb/acre each of Greenfeed and urea applied on 22 July, 20 lb/acre as Greenfeed on 29 July and 12 August. xSidedress N source: urea, applied at 60 lb/acre on 22 July, 40 lb/acre on 29 July, and 40 lb/acre on 12 August. wSidedress N source: urea, applied at 40 lb/acre on 22 July, 40 lb/acre on 29 July, and 20 lb/acre on 12 August. vSidedress N source: urea, applied at 20 lb/acre, each, on 22 July, 29 July, and 12 August. Table 14. Sweet corn yield and soil and stalk nitrate-N concentrations as affected by N rate, on-farm PSNT trial, 1997. N fertilizer Soil nitrate-N Stalk Corn Total rate Sidedress Preplant PSNT Harvest nitrate-N yield Grower ------lb/acre------- --------------ppm---------------------- tons/acre A 130 70 9.1 13.5 4.9 3010 9.48 A 130 70 9.1 10.9 4.5 10000 9.09 A 130 70 9.1 8.8 4.5 9010 8.84 A 200 140 9.1 13.1 11.0 9550 9.53 A 200 140 9.1 10.0 11.7 11180 9.05 A 200 140 9.1 12.2 8.9 11880 9.42 B 135 85 22.0 33.3 20.1 13080 9.24 B 230 190 22.0 32.6 34.1 12120 9.31 C 116 64 38.2 35.4 8.9 2398 8.96 C 180 123 8.2 38.8 19.0 7129 8.89 D 140 80 7.3 15.6 7.5 8130 7.17 D 220 160 7.3 13.1 7.3 6250 6.97 E 140 88 10.4 22.9 8.3 8545 10.30 E 140 88 10.4 23.5 8.6 5801 8.94 E 140 88 10.4 25.4 14.0 6935 8.95 E 200 148 10.4 23.2 21.6 7501 10.59 E 200 148 10.4 26.7 21.5 7161 9.63 E 200 148 10.4 27.0 12.7 7143 9.35 F 107 56 13.8 23.6 6.5 4926 10.48 F 107 56 13.8 22.9 7.2 4035 10.28 F 107 56 13.8 22.5 8.5 3144 10.07 F 145 94 13.8 21.2 13.2 4003 10.72 F 145 94 13.8 23.3 5.9 2904 10.08 F 145 94 13.8 21.1 13.2 2983 10.67 G 120 80 14.4 22.5 6.4 5372 10.65 G 200 160 14.4 19.9 2.5 10275 12.10 H 130 64 8.5 15.9 1.7 223 11.34 H 195 129 8.5 15.0 5.3 1574 11.83 I 140 90 1.4 12.2 12.1 7440 8.08 I 227 177 1.4 24.7 24.7 10950 9.66 Table 15. Sidedress N rate for sweet corn based on 1995?1996 PSNT trials. PSNT Sidedress N ppm lb/acre <18 150 18 to 22 100 >22 50 to 80 Table 16. Fertilizer, PSNT, yield, and soil and corn stalk nitrate-N concentrations in grower fields, 1998. Grower N at PSNT PSNT N applied PSNT plus Yield Relative Nitrate at harvest planting sidedress sidedress N yield Stalk Soil lb/acre ppm lb/acre lb/acre lb/acre tons/acre % ppm ppm A field 40 43 155 90 245 10.52 -- 20 plot 40 35 125 45 170 10.82 102.9 -- 19 B field 66 27 97 116 213 13.47 12916 17 plot 66 27 97 88 185 13.98 103.8 11729 19 C field 40 32 115 40 155 11.56 11637 22 plot 40 32 115 25 140 10.00 86.5 11921 41 D field 47 15 53 155 208 11.22 3014 11 plot 47 17 60 120 180 10.58 94.3 2370 12 E field 50 25 88 160 248 10.05 7306 48 plot 50 24 85 80 165 9.28 92.3 7015 20 F field 39 39 142 120 262 11.72 9990 37 plot 39 35 126 48 174 11.64 99.3 10890 41 G field 45 33 117 160 277 7.55 2461 19 plot 45 37 133 0 133 7.24 95.9 4047 52 H field 50 22 79 150 229 10.44 8197 17 plot 50 21 76 100 176 9.67 92.6 6637 10 I field 30 44 160 75 235 8.63 8923 67 plot 30 43 155 50 205 8.48 98.3 8117 23