Vegetable growers in the Willamette Valley use high rates of nitrogen 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. This has raised concerns that the remaining N may be contributing to nitrate pollution of groundwater. Improved efficiency of nitrogen management may be possible if the fertilizer could be placed for maximum contact with the root system.
Trials conducted in 1989 through 1994 indicated that broccoli is more efficient than is sweet corn in recovering applied N, even though higher rates of N are typically applied to broccoli. This may be due to differences in root architecture but between-row spacing may also play a role. Sweet corn is typically grown at row spacings of 30 or 36 inches. Broccoli is most commonly grown at spacings that average 20 inches. Therefore, we may deduce that broccoli roots might explore a greater proportion of the space between rows. Broccoli is also typically planted at higher populations than is sweet corn. The purpose of this trial was to investigate whether decreasing row spacing, but maintaining a constant population by increasing in-row spacing, would increase N uptake efficiency in sweet corn, as measured by the amount of mineral N remaining in the soil at harvest.
'Jubilee' sweet corn was seeded in a Willamette silt loam, pH 5.9, at the NWREC on 24 May using a hand-push garden planter. Plot preparation included a broadcast and incorporated application of 10N-8.3P-16.7K (10-20-20 fertilizer) at 500 pounds per acre, disking, and cultimulching. Fifty pounds N/acre as urea was applied to all plots immediately after planting. Plot size was 15 feet wide by 30 feet long. Spacing between rows was 18, 27, or 36 inches. Immediately after planting, atrazine was applied at 2.0 pounds/acre and alachlor at 3.0 pounds/acre for weed control.
During the third week of June corn stands were thinned to a nominal population of 25,000/acre. The remaining N was broadcast to the appropriate plots on 22 June (split application). The six combinations of the two fertilizer rates and three spacings were in randomized complete block design with four replications. The plots were sprinkler-irrigated as necessary and harvested on 24 August.
After harvest, the stover was mowed and left in place on the plots. The plots were sampled to a depth of 2 feet in 1-foot increments on 27 September. The samples were submitted to the OSU Central Analytical Laboratory for determination of residual nitrate and ammonium concentration.
Results and Discussion
The effect of row spacing was tested both at an optimal and at a suboptimal rate of N. Yield, mean ear weight, ear length, and residual nitrate levels were greater at 200 than at 50 pounds applied N/acre (Tables 1 and 2). Only main effects are shown in the tables as there were no significant interactions of N rate and row spacing affecting any yield parameter or residual N. Seeding with the garden planter was not entirely satisfactory and stands were somewhat less than the desired 25,000/acre, particularly for the 18- and 36-inch spacings. Thus yield tended to be slightly greater at the 27-inch spacing, but the effect of spacing on yield was not signficant (Table 1). The effect of spacing on residual soil nitrate and ammonium concentrations was also not significant (Table 2), giving no support to our hypothesis that closer row spacings might result in more efficient N uptake and reduced residual mineral N. However, residual N at 200 pounds applied N/acre was unusually low in this experiment compared to many of our trials.
Table 1. Main effects of rate of urea-nitrogenz and row spacing on the yield of sweet corn, NWREC, 1995 Treatment Yield Ear wt. Ear length (T/A) (g) (inches) N rate (lb/acre) 50 4.5 220 10.5 200 9.0 283 11.6 Significance ** ** ** Row spacing (inches) 18 6.5 266 11.1 27 7.4 232 10.9 36 6.4 257 11.1 Significance NS NS NS zFifty pounds N/acre broadcast at planting, remainder broadcast five weeks later. **, NS: Significant at p=0.01 and nonsignificant, respectively. Table 2. Main effects of N rate and row spacing on post-harvestz soil nitrate and ammonium concentrations, 27 September, 1995 Treatment Nitrate-N Ammonium-N 0-12 inches 12-24 inches 0-12 inches 12-24 inches N rate (lb/acre) -------------------------ppm------------------------- 50 0.2 0.2 3.0 2.9 200 10.9 2.3 4.2 3.5 Significance * * NS NS Row spacing (inches) 18 4.7 1.6 3.6 3.2 27 7.2 1.1 3.7 3.5 36 4.7 1.1 3.6 2.8 Significance NS NS NS NS zPre-plant nitrate levels were 0.2 ppm at both 0-12 and 12-24 inch depths. Pre-plant ammonium levels were 2.8 and 3.2 ppm at 0-12 and 12-24 inch depths, respectively. *,NS: significant at p=0.05 and nonsignificant, respectively.