Introduction
The rationale for this trial was similar to that of the previously described broccoli and sweet corn experiments. Sweet corn is planted at smaller populations and wider row spacings than is broccoli and may be less effective at taking up applied N. The purpose of this trial was to determine if yield of sweet corn would be affected by placement or source of N fertilizer at several rates of applied N.
Methods
'Jubilee' sweet corn was seeded in a Willamette silt loam, pH 5.9, at the NWREC on 19 May, 1992. Plot preparation included a broadcast and incorporated application of triple superphosphate at 250 pounds/acre and potassium sulfate at 250 pounds/acre, disking and cultimulching. Another 60 pounds/acre of triple superphosphate was banded 2 inches to the side and 2 inches beneath the seed row on all plots. Forty pounds N/acre as urea, ammonium nitrate, calcium-ammonium nitrate (CAN-17), or urea-ammonium nitrate (UAN-32) were also shanked in at 2 inches beneath and 2 inches to the side of the seed row on all but the zero N and zero N at planting treatments (Table 1). The pelleted urea and ammonium nitrate were mixed with 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 seedlings were thinned to approximately 7 inches between plants in the row on 9 June. The remaining nitrogen was shanked in or broadcast to the appropriate plots on 29 June. 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 17 August.
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 26 August and their identity was maintained over the winter so that samples could be taken in the spring of 1993.
Results and Discussion
When all the sidedressed nitrogen fertilizer was banded as urea, yield increased with increasing rate of N to a maximum at 180 pounds N/acre (Table 2). However, the yields at 120 and 240 pounds N/acre were not significantly different than at 180 pounds/acre. Mean ear weight and length tended to be greatest at 120 pounds N per acre.
All other combinations of N source and application method were at 60 and 180 pounds total N/acre. Comparisons of N utilization are based on banded urea as the standard. Mean yield per unit area of corn fertilized at 60 and 180 pounds N/acre did not vary significantly with N source (Table 3). However, the plants fertilized with UAN-32 were consistently poorest in terms of number of ears harvested, mean ear weight and length, and degree of tipfill of the ear. Although not statistically significant, yield also tended to be lowest with UAN-32. The greatest number of ears harvested and the greatest yield were with CAN-17 as N source, but the greatest mean ear weight, length, and tipfill were with ammonium nitrate.
When comparing only sweet corn fertilized with urea or ammonium nitrate and averaged over method of placement of the sidedressed fertilizer, ammonium nitrate appeared slightly superior to urea, but the difference was significant only for mean ear weight (Table 4). Past research at NWREC with urea, ammonium nitrate, and other solid N sources indicated no consistent differences among nitrogen sources in effects on corn yields. When comparing a broadcast versus a banded application of sidedressed fertilizer (Table 4), there were no significant effects on yield or quality.
Sweet corn production with zero or 60 pounds/acre of applied N reduced nitrate concentration in the top 30 inches of the soil profile during the growing season (Table 5). Soil ammonium concentration was not greatly affected by sweet corn fertilized with these same rates. However, at 120 or more pounds N/acre, soil nitrate and ammonium concentrations were greatly elevated in the surface 10 inches of soil. This is in contrast to soil cropped with broccoli where in 1991, 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. Although not statistically significant, there was one apparent exception. Both nitrate and ammonium levels tended to increase at the 10 to 20-inch depth at the 120 pound/acre rate of urea. In each case, only two of the four replicates of the samples showed unusually high levels of nitrate and ammonium.
The high levels of residual fertilizer present at rates of N needed for acceptable sweet corn 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 N than is broccoli. This indicates the need for more research on improving N uptake efficiency in sweet corn.
Table 1. List of N application treatments, 1992 sweet corn nitrogen utilization trial, NWREC No. Total N rate N source Rate at planting Sidedress rate and method (lb/A) (lb/A) (lb/A) 1 0 None 0 0 2 60 Urea 40 20 banded 3 120 Urea 40 80 banded 4 180 Urea 40 140 banded 5 240 Urea 40 200 banded 6 60 NH4NO3 40 20 banded 7 180 NH4NO3 40 140 banded 8 60 CAN-17 40 20 banded 9 180 CAN-17 40 140 banded 10 60 UAN-32 40 20 banded 11 180 UAN-32 40 140 banded 12 60 Urea 40 20 broadcast 13 180 Urea 40 140 broadcast 14 60 NH4NO3 40 20 broadcast 15 180 NH4NO3 40 140 broadcast 16 180 Urea 0 180 banded Table 2. Yield, ear size, and tipfill of sweet corn as affected by rate of banded urea, NWREC, 1992 N rate No. ears/ Yield Mean ear Mean ear Tipfillz (lb/acre) acre (T/acre) wt. (g) length (cm) 0 13,830 3.4 214 22.3 2.3 60 24,070 7.6 278 22.9 2.6 120 27,880 9.3 292 23.8 2.5 180 32,230 10.1 274 22.7 2.6 240 30,060 9.7 287 23.6 2.6 LSD(0.05) 5,920 1.7 29 1.0 NS zA measure of the degree of kernel formation at the silk end of the ear. Five point scale with 5 = kernels filled out to tip of ear, 1 = at least 5 cm of ear without filled kernels. Table 3. Yield, ear size, and tipfill of sweet corn as affected by rate and source of banded nitrogen fertilizer, NWREC, 1992 N source N rate No. ears/ Yield Mean ear Mean ear Tipfill (lb/acre) acre (T/acre) wt. (g) length (cm) Urea 60 24,070 7.6 278 22.9 2.6 180 32,230 10.1 274 22.7 2.6 Mean 28,150 8.9 276 22.8 2.6 NH4NO3 60 27,010 8.7 279 22.6 2.8 180 28,530 9.8 298 23.9 2.7 Mean 27,770 9.2 288 23.2 2.7 CAN-17 60 29,510 8.6 253 22.7 2.7 180 30,600 10.3 294 23.2 2.3 Mean 30,050 9.5 274 23.0 2.5 UAN-32 60 21,160 7.6 263 22.0 2.3 180 28,210 9.6 265 22.9 2.1 Mean 24,690 8.6 264 22.5 2.2 LSD(0.05), means 4,890 NS 22 0.7 0.4 Table 4. Yield, ear size, and tipfill of sweet corn as affected by rate and method of placement of sidedressed nitrogen fertilizer, NWREC, 1992 Source Placement N rate No. ears/ Yield Mean ear Mean ear Tipfill (lb/acre) acre (T/acre) wt. (g) length (cm) Urea Band 60 24,070 7.6 278 22.9 2.6 180 32,230 10.1 274 22.7 2.6 Mean 28,150 8.9 276 22.8 2.6 Broadcast 60 27,990 8.0 246 22.1 2.5 180 29,190 9.6 288 23.4 2.8 Mean 28,590 8.8 267 22.8 2.6 NH4NO3 Band 60 27,010 8.7 279 22.6 2.8 180 28,530 9.8 298 23.9 2.7 Mean 27,770 9.2 288 23.2 2.7 Broadcast 60 26,680 8.3 279 22.5 2.8 180 30,710 9.9 282 23.2 2.7 Mean 28,700 9.1 281 22.9 2.7 Urea mean 28,370 8.8 272 22.8 2.6 NH4NO3 mean 28,240 9.2 285 23.1 2.7 Significance, N source NS NS * NS NS Banded mean 27,960 9.1 282 23.0 2.7 Broadcast mean 28,650 9.0 274 22.8 2.7 Significance, placement NS NS NS NS NS Table 5. Effect of rate of banded urea on post-harvest soil nitrate and ammonium concentrations, NWREC, 26 August, 1992 Sample depth Rate of applied urea, lb/acre LSD (0.05) Pre-plant (inches) 0 60 120 180 240 ----------------------ppm--------------------------- Nitrate 0-10 0.5 1.1 22.6 27.1 48.2 24.6 5.6 10-20 0.1 0.9 20.0 2.9 4.8 NS 3.8 20-30 0.3 1.0 5.7 1.6 2.1 NS 3.1 30-40 2.5 2.2 2.2 3.0 3.7 NS 3.8 Ammonium 0-10 2.9 2.5 30.2 106.7 56.2 49.0 4.8 10-20 2.8 2.5 8.4 6.5 6.0 NS 2.8 20-30 3.0 2.7 3.3 3.7 3.6 NS 5.1 30-40 2.7 2.8 3.3 7.0 3.1 NS 4.4