Introduction
Vegetable growers in the Willamette Valley use high rates of N fertilizers, often exceeding 250 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. Our research has shown that broccoli needs 250 pounds N/acre for maximum yield. At this N rate very little nitrate-N remained in the root zone. The same was true of cauliflower. However, when sweet corn was fertilized for maximum yield, significant quantities of applied N were not taken up by the crop.
A survey of grower fields was instituted in 1993, where 30 fields were sampled for nitrate and ammonium-N concentrations before spring fertilization and were then cropped to beans, beets, broccoli, carrots, cauliflower, and sweet corn. At the end of the growing season, the same fields were tested for residual nitrate and ammonium concentration. Residual levels were correlated with amounts of N applied and other grower cultural practices. This survey was repeated in 1994 and 1995 on 34 fields. To provide a basis of comparison with the grower fields, crop yield and residual mineral N were measured as a function of applied N for beans, beets, carrots, and sweet corn at NWREC. The sweet corn trials involved other factors such as timing of N application and results are presented elsewhere. The bean, beet, and carrot results are presented here.
Methods
All crops were seeded to a Willamette silt loam, pH 6.1, at NWREC. Plot preparation for all crops included a broadcast and incorporated application of potassium sulfate at 250 pounds/acre and triple superphosphate at 150 pounds/acre, disking and cultimulching. Pre-plant soil samples were obtained to 4-foot depth, in 1-foot increments, on 27 April. The samples were submitted to the OSU Central Analytical Lab for analysis of nitrate and ammonium content. The N source for all trials was urea. Post-harvest soil samples to 2-foot depth were obtained on 21 September. Irrigation of all crops ceased on the harvest date to minimize nitrate leaching and further N uptake by the crop.
Beans
The plot area received a broadcast, incorporated application of 0.75 pounds trifluralin, 2.0 pounds EPTC, and 1.2 pounds chlorpyrifos/acre. 'Oregon 91G' snapbeans were seeded at 65 pounds/acre on 15 May with three rows per plot on 20-inch centers. The first 40 pounds N/acre were broadcast on the planting date; the remaining N was broadcast on 16 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 25 July.
Beets and Carrots
The plot area received a broadcast, incorporated application of EPTC at 2.0 pounds/acre (beets). Carrot plots were treated with linuron at 1.25 pound/acre one day after seeding. 'Detroit Dark Red' table beet and 'Six Pac' carrot were seeded on 15 May with three rows on a 5-foot bed. The first 40 pounds N/acre were broadcast on the planting date; the remaining N was broadcast on 10 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 8 August (beets) and 13 September (carrots).
Results and Discussion
Bean yield was highest at 80 pounds N/acre (Table 17). As in 1994, carrot yield was highest at 120 pounds N/acre (Table 17). Carrot yields were lower in 1995 than in 1994, reflecting, in part, siting the experiment on Willamette rather than Latourell soil. Also as in 1994, beet yield increased markedly with the first increment of applied N and continued to increase in quadratic fashion with the maximum at 240 pounds N/acre (Table 18). This is also consistent with previous research at NWREC and with grower experience. Table beet yield response to N rate was nearly identical in 1994 and 1995.
Nitrate concentration in the surface foot of soil after snapbean harvest increased dramatically between the rates of 80 and 120 pounds applied N/acre (Table 19). Soil ammonium content also increased with increasing applied N for the surface foot of soil. The largest increase came between the rates of 120 and 160 pounds applied N/acre (Table 19). Rate of applied N did not significantly affect soil nitrate or ammonium content at the 1-2 foot depth.
Soil nitrate concentration in the surface foot of soil increased with increasing rate of applied N for both beet and carrot (Tables 20 and 21). In the second foot of soil, the increase was significant only for carrots. Residual mineral N following beet was low compared to most crops. For carrots, there was not much increase in soil nitrate content in the surface foot of soil until the applied N reached the optimal level for yield (Table 21). This is consistent with 1994 results. Soil ammonium content appeared to increase at both depths following the higher rates of applied N for table beets. However, the increases were not statistically significant. Soil ammonium content in the surface foot of soil increased slightly, but significantly, with the highest rate of N applied to carrots.
Table 17. Effect of rate of urea-nitrogen on the yield of green beans and carrots, NWREC, 1995 N rate Bean Carrot (lb/A) (T/A) (T/A) 0 2.9 10.9 40 3.9 14.7 80 7.3 15.9 120 6.1 19.6 160 6.8 18.0 Significance L*Q* L*Q* L=linear, Q=quadratic, *significant, p=0.05. Table 18. Effect of rate of urea-nitrogen on the yield of table beets, NWREC, 1995 N rate Yield (lb/A) (T/A) 0 3.6 60 14.1 120 14.8 180 17.5 240 19.8 Significance L**Q* Table 19. Effect of rate of broadcast N on soil nitrate and ammonium concentrations (ppm N) at two depths following bean harvest, NWREC, 1995 Sample depth N rate, lb/A (inches) 0 40 80 120 160 LSD(0.05) Pre-plant ---------Post-harvest-------- Nitrate 0-12 0.2 5.5 7.3 7.5 17.1 13.8 6.1 12-24 0.2 1.4 1.9 2.0 2.0 3.4 NSD Ammonium 0-12 3.6 3.1 3.1 3.8 5.6 8.4 1.7 12-24 3.1 3.2 3.1 3.3 3.6 4.6 NSD Table 20. Effect of rate of broadcast N on soil nitrate and ammonium concentrations (ppm N) at two depths following beet harvest, NWREC, 1994 Sample depth N rate, lb/A (inches) 0 40 80 120 160 LSD(0.05) Pre-plant ---------Post-harvest-------- Nitrate 0-12 0.2 0.9 1.3 4.6 6.6 7.7 4.2 12-24 0.2 0.5 1.1 0.8 1.0 2.6 NSD Ammonium 0-12 3.6 2.9 3.3 5.8 10.2 18.2 NSD 12-24 3.1 3.2 2.8 3.5 5.0 8.2 NSD Table 21. Effect of rate of broadcast N on soil nitrate and ammonium concentrations (ppm N) at two depths following carrot harvest, NWREC, 1995 Sample depth N rate, lb/A (inches) 0 40 80 120 160 LSD(0.05) Pre-plant ---------Post-harvest-------- Nitrate 0-12 0.2 1.0 1.1 4.7 14.5 21.0 4.0 12-24 0.2 0.2 1.5 1.1 2.7 3.3 1.8 Ammonium 0-12 3.6 3.5 3.1 3.6 3.4 5.8 1.3 12-24 3.1 2.8 3.5 2.8 3.0 3.2 NSD