Introduction
The justification for this trial is similar to that for cauliflower, reported earlier. A sweet corn trial in 1992 indicated that source and placement of nitrogen fertilizer had little effect on yield or quality of sweet corn. Rates of nitrogen application greater than 60 pounds per acre resulted in accumulation of significant amounts of nitrate-N in the soil. The purpose of these trials was to confirm 1992 results and to determine if yield of sweet corn would be affected by source or timing of application of nitrogen fertilizer.
Methods
'Jubilee' sweet corn was seeded into a Willamette silt loam, pH 5.9, at the NWREC on 13 May, 1993, and 25 May, 1994. Plot preparation included a broadcast and incorporated application of potassium sulfate at 250 pounds/acre, disking and cultimulching. Triple superphosphate was banded at 130 pounds/acre, two inches to the side and two inches beneath the seed row on all plots. Forty pounds of nitrogen/acre as urea, ammonium nitrate, calcium-ammonium nitrate (CAN-17), or urea-ammonium nitrate (UAN-32) was also shanked in at 2 inches beneath and 2 inches to the side of the seed row on all but the zero nitrogen treatment (Table 18). The prilled urea and ammonium nitrate were applied in the same band as the superphosphate. The liquid CAN-17 and UAN-32 were applied with separate shanks mounted behind the superphosphate shanks.
Plot size was 15 feet wide (six rows) by 30 feet long. Spacing between rows was 30 inches. Immediately after planting atrazine was applied at 2.0 pounds/acre and alachlor at 3.0 pounds/acre. The remaining nitrogen was shanked in (banded) or broadcast to the appropriate plots at planting or on 21 June, 1993 or 29 June, 1994 (split application). Treatments consisting of various rates, sources, and sidedress application methods were in randomized complete block design with four replications.
The plots were sprinkler-irrigated as necessary and harvested on 24 August, 1993 and 2 September, 1994. Following completion of harvest, the stover was mowed and left in place on the plots. The plots were sampled for residual soil nitrate and ammonium concentration on 22 October, 1993, before the onset of fall rains, and their identity was maintained over the winter so that samples could be taken in the spring of 1994.
Results and Discussion
When all the sidedressed nitrogen fertilizer was banded as urea (Treatments 1-5), yield increased with increasing rate of N to a maximum at 180 pounds N/acre in both years (Table 19). However, the yields at 120 and 240 pounds N/acre were not significantly different than at 180 pounds/acre. Mean ear weight, number of ears harvested, ear length, and tipfill also tended to be greatest at 180 or 240 pounds N/acre, but there were no significant differences among the three greatest rates of N. Kernel moisture content was approximately 73 percent for all treatments.
The other combinations of N source and application method were at 120 pounds total N/acre in 1993 and 120 or 180 pounds N/acre in 1994. Comparisons of N utilization are based on banded urea at planting and broadcast urea sidedress, with a split application, as the standard. Mean yield of corn fertilized at 120 pounds N/acre did not vary significantly with nitrogen source in either year (Tables 20 and 21). This is consistent with results obtained in 1992. Past research at NWREC with urea, ammonium nitrate, and other solid nitrogen sources indicated no consistent differences among nitrogen sources in effects on corn yields.
When comparing the effect of the timing of the sidedressed N application, and averaged over urea and ammonium nitrate as N source, a split or delayed application of the sidedressed N appeared slightly superior to applying all fertilizer at planting for yield, number of ears harvested, ear weight, and tipfill, but the differences were not significant in either year (Table 22). An effect of split application might have been expected in 1993 because of the greater than normal precipitation (15.3 cm) and, thus, leaching potential, during the interval between planting and the delayed sidedress application. However, in 1994, precipitation was only 4.5 cm during the interval between planting and the delayed sidedress, indicating little potential for leaching.
In a 1993 comparison of broadcast versus banded application of sidedressed urea or ammonium nitrate fertilizer (Table 23), there were no significant effects on yield or quality. However, the number of ears/acre was greater with broadcast application of the sidedressed nitrogen.
In comparing CAN-17 with urea as N source, at 180 pounds N/acre, CAN-17 again tended to produce greater yield, but the difference was not significant (Table 24).
Sweet corn production with zero to 120 pounds applied N/acre effectively reduced nitrate concentration in the first 40 inches of the soil profile during the growing season of 1993 (Table 25). However, at 180 or more pounds N/acre, soil nitrate concentrations were greatly elevated in the surface 10 inches of soil. Soil ammonium concentrations were not greatly affected by sweet corn fertilized with any rate of N. These results are fairly consistent with those obtained in 1992, when levels of both nitrate and ammonium were greatly increased in the surface layer of soil by rates as low as 120 lb N/acre. This is in contrast to soil cropped with broccoli: in 1991 and 1992 rates of nitrogen up to 250 pounds/acre did not increase nitrate and ammonium levels beyond those present at planting. There is very little indication in this experiment of movement of applied nitrogen beyond the root zone. Increased nitrate and ammonium levels were generally confined to the surface 10 inches.
Plots were again sampled for nitrate and ammonium content in the spring of 1994, following approximately 24 inches of precipitation (Table 26). Ammonium concentration did not vary with rate of applied N. Soil nitrate concentration again did not vary with applied N from 0 to 120 pounds N/acre. However, at the higher rates of N, nitrate concentration remained elevated, although only at depths greater than 20 inches. This is indirect evidence for the leaching of nitrate from the surface 20 inches of soil toward the vadose zone.
In contrast to 1992 and 1993, rate of applied N had relatively little effect on post-harvest soil concentrations of ammonium and nitrate in 1994 (Table 27). Soil nitrate content tended to be slightly elevated in the surface 12 inches of soil at the optimal N rate of 180 pounds/acre, but the effect was not significant and there was no effect at all at suboptimal rates of N. At the greatest rate of applied N, soil nitrate, but not ammonium levels, were significantly increased, but to a lesser extent than in the previous years. Any effect of N rate at greater depth was masked by the greater-than-normal nitrate concentrations that existed at time of planting. There was a small but significant effect of treatment on soil ammonium concentration at 36 to 48 inches. The form of applied N had no effect on soil nitrate concentrations at harvest, but there was, again, a small effect on soil ammonium content at 36 to 48-inch depth.
The high levels of residual fertilizer present at rates of nitrogen needed for acceptable yields is in contrast to the situation for broccoli and is a cause for concern. Apparently sweet corn is less efficient at taking up applied nitrogen than is broccoli. Measurements of the nitrogen content of the above-ground biomass of each crop support this conclusion. This indicates the need for more research on improving nitrogen uptake efficiency in sweet corn.
Table 18. List of N application treatments, sweet corn nitrogen utilization trial, NWREC No. N rate N source Banded at Broadcast at Sidedress rate, methodz (lb/A) seeding (lb/A) seeding (lb/A) (lb/A) 1993 1 0 None 0 0 0 2 60 Urea 40 0 20 broadcast 3 120 Urea 40 0 80 broadcast 4 180 Urea 40 0 140 broadcast 5 240 Urea 40 0 200 broadcast 6 120 NH4NO3 40 0 80 broadcast 7 120 CAN-17 40 0 80 banded 8 120 UAN-32 40 0 80 banded 9 120 Urea 40 80 0 10 120 Urea 40 0 80 banded 11 120 NH4NO3 40 80 0 12 120 NH4NO3 40 0 80 banded 13 180 CAN-17 40 0 140 banded 1994 1 0 None 0 0 0 2 60 Urea 40 0 20 broadcast 3 120 Urea 40 0 80 broadcast 4 180 Urea 40 0 140 broadcast 5 240 Urea 40 0 200 broadcast 6 120 NH4NO3 40 0 80 broadcast 7 120 CAN-17 40 0 80 banded 8 120 UAN-32 40 0 80 banded 9 120 Urea 40 80 0 10 120 NH4NO3 40 80 0 11 180 Urea 40 140 0 12 180 NH4NO3 40 140 0 13 180 NH4NO3 40 None 140 broadcast 14 180 CAN-17 40 None 140 dribble band 15 180 UAN-32 40 None 140 dribble band zNitrogen sidedressed on 21 June, 1993 and 29 June, 1994. Table 19. Effect of rate of urea-nitrogenz on the yield of sweet corn, NWREC, 1993 and 1994 N rate Yield No. ears Ear wt. Ear length Tipfilly (lb/A) (T/A) per acre (g) (inches) 1993 0 2.7 10680 241 8.2 2.9 60 5.8 20060 268 8.7 2.9 120 7.5 25400 268 8.9 3.1 180 7.7 25960 272 8.8 3.3 240 7.5 25510 268 9.0 3.4 LSD (0.05) 1.5 6330 NSD 0.3 0.4 1994 0 7.2 37020 198 8.6 3.0 60 10.6 37333 263 9.4 3.8 120 11.6 38990 271 9.3 3.9 180 12.5 42040 275 9.5 4.2 240 10.5 35830 268 9.4 3.8 LSD (0.05) 2.1 NS 50 0.3 0.5 zForty pounds N/acre banded at planting, remainder broadcast five weeks later. yFive-point scale with 5=perfect fill. Table 20. Effect of four nitrogen sources, at 120 pounds N/acrez, on the yield of sweet corn, NWREC, 1993 and 1994 N source Yield No. ears Ear wt. Ear length Tipfill (T/A) per acre (g) (inches) 1993 Urea 7.1 22670 286 8.9 3.3 NH4NO3 7.0 23980 271 8.9 3.5 CAN-17 7.3 24960 267 8.9 3.3 UAN-32 6.7 22560 269 8.7 3.2 LSD (0.05) NS NS NS NS NS 1994 Urea 11.6 38990 271 9.3 3.9 NH4NO3 11.3 33210 307 9.5 4.1 CAN-17 11.2 39970 267 9.4 4.1 UAN-32 11.1 40030 266 9.2 3.7 LSD (0.05) NS NS NS NS NS zForty pounds N/acre banded at planting, 80 pounds N/acre banded five weeks later. Table 21. Effect of four nitrogen sources, at 180 pounds N/acrez, on the yield of sweet corn, NWREC, 1994 N source Yield No. ears Ear wt. Ear length Tipfill Moisture (T/A) per acre (g) (inches) (%) Urea 12.5 42040 275 9.5 4.2 71.3 NH4NO3 12.2 39750 284 9.3 4.2 71.4 CAN-17 11.7 42250 253 9.5 3.8 74.2 UAN-32 10.6 37350 256 9.4 4.1 73.6 LSD (0.05) 1.6 NS NS NS NS NS zForty pounds N/acre banded at planting, 140 pounds N/acre banded five weeks later. Table 22. Interaction of nitrogen source and timing of sidedress nitrogenz application on the yield of sweet corn, NWREC, 1993 and 1994 N source Timing Yield No. ears Ear wt. Ear length Tipfill (T/A) per acre (g) (inches) 1993 Urea planting 7.0 24310 265 8.9 3.0 Urea 5 weeks 7.5 25400 268 8.9 3.1 NH4NO3 planting 7.3 24850 269 8.8 3.1 NH4NO3 5 weeks 7.7 26270 271 8.8 3.3 NS NS NS NS NS 1994 Urea planting 11.2 38880 264 9.3 3.9 Urea 5 weeks 12.0 40510 273 9.4 4.1 NH4NO3 planting 11.2 35070 292 9.4 4.2 NH4NO3 5 weeks 11.8 36480 295 9.4 4.2 LSD (0.05) NS NS 21 NS NS zRate of nitrogen = 120 pounds/acre in 1993 with 80 pounds/acre sidedressed. Mean of nitrogen applications of 120 and 180 pounds/acre in 1994. Sidedress application average of 80 and 140 pounds/acre in 1994. Table 23. Effect of banded versus broadcast sidedress nitrogenz application on sweet corn yield, NWREC, 1993 N source Sidedress Yield No. ears Ear wt. Ear length Tipfill method (T/A) per acre (g) (inches) Urea Band 7.1 22670 286 8.9 3.3 Urea Broadcast 7.5 25400 268 8.9 3.1 NH4NO3 Band 7.0 23980 271 8.9 3.5 NH4NO3 Broadcast 7.7 26270 271 8.8 3.3 LSD (0.05) NS 2950 NS NS NS zRate of applied nitrogen = 120 pounds/acre. Sidedressed five weeks after planting. Table 24. Effect of CAN-17 versus ureaz as N source on the yield of sweet corn, NWREC, 1993 N source Yield No. ears Ear wt. Ear length Tipfill (T/A) per acre (g) (inches) CAN-17 8.3 28890 260 9.0 3.3 Urea 7.7 25980 272 8.8 3.3 LSD (0.05) NS NS NS NS NS zNitrogen applied at 180 pounds/acre. Table 25. Effect of rate of nitrogen on post-harvest soil nitrate and ammonium concentrations, 25 October, 1993 Sample depth Rate of applied urea, lb/acre LSD (0.05) Pre-plant (inches) 0 60 120 180 240 ----------------------ppm------------------------ Nitrate 0-10 2.6 2.9 2.9 7.1 22.2 4.7 3.6 10-20 1.3 1.0 1.4 2.1 7.0 3.1 2.2 20-30 1.4 2.2 1.8 2.3 4.0 NS 2.3 30-40 1.9 1.9 2.4 2.7 3.1 NS 3.1 Ammonium 0-10 2.6 2.1 1.9 3.3 2.9 NS 3.1 10-20 1.9 1.6 1.3 1.8 2.0 NS 3.6 20-30 1.8 2.0 1.9 2.0 2.1 NS 1.9 30-40 1.9 1.7 2.2 2.4 1.9 NS 2.3 Table 26. Effect of rate of nitrogen on residual soil nitrate and ammonium concentrations, 28 April, 1994 Sample depth Rate of applied urea, lb/acre LSD (0.05) (inches) 0 60 120 180 240 -----------------ppm-------------------- Nitrate 0-10 1.2 0.9 1.4 0.9 1.6 NS 10-20 1.5 1.5 2.2 2.1 2.9 NS 20-30 1.7 1.7 2.8 3.9 5.2 1.2 30-40 1.6 1.6 2.6 4.6 6.1 2.5 Ammonium 0-10 3.1 2.7 3.0 3.1 3.3 NS 10-20 2.6 2.5 2.5 2.3 2.3 NS 20-30 3.1 2.3 2.6 2.3 2.2 NS 30-40 2.7 2.1 2.5 2.5 2.4 NS Table 27. Effect of rate of nitrogen on post-harvest soil nitrate and ammonium concentrations, 9 September, 1994 Sample depth Rate of applied urea, lb/acre LSD (0.05) Pre-plant (inches) 0 60 120 180 240 ----------------------ppm------------------------- Nitrate 0-12 0.3 0.4 0.8 4.6 10.9 8.6 0.6 12-24 1.4 3.0 1.5 8.7 7.9 NS 8.2 24-36 9.2 7.6 3.8 9.7 6.2 NS 14.0 36-48 8.7 8.7 5.0 7.1 8.0 NS 2.3 Ammonium 0-12 2.1 2.2 2.5 2.2 5.4 NS 2.1 12-24 2.1 2.4 2.8 2.2 5.2 NS 1.8 24-36 2.5 2.3 2.9 2.3 6.5 NS 1.8 36-48 2.0 2.2 2.4 1.6 4.0 1.4 1.6 Table 28. Effect of nitrogen source on post-harvest soil nitrate and ammonium concentrations at 180 pounds applied N/acre, 9 September, 1994 N source Sample depth (inches) Urea NH4NO3 CAN-17 UAN-32 LSD (0.05) ------------------ppm------------------- Nitrate 0-12 4.6 0.4 0.8 4.6 NS 12-24 8.7 3.0 1.5 8.7 NS 20-36 9.7 7.6 3.8 9.7 NS 36-48 7.1 8.7 5.0 7.1 NS Ammonium 0-12 2.2 3.5 3.0 3.8 NS 12-24 2.2 3.0 3.5 2.6 NS 24-36 2.3 2.8 2.8 2.8 NS 36-48 1.6 3.5 2.7 2.5 1.4