Introduction
A survey of grower fields was initiated in 1993, in which 30 fields were sampled for nitrate and ammonium-N concentrations before fertilization and were then cropped to beans, beets, broccoli, carrots, cauliflower, and sweet corn. At the end of the growing season, the fields were tested for residual nitrate and ammonium concentration. This survey was repeated in 1994 on 34 fields. To provide a basis of comparison with the grower fields, crop yield and residual mineral N were measured in NWREC trials as a function of applied N for beans, beets, carrots, cauliflower, and sweet corn. The cauliflower and sweet corn trials involved other factors such as N source and 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 per acre, disking and cultimulching. Pre-plant soil samples were obtained to four-foot depth, in one-foot increments, on 21 April. The samples were frozen and 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 were obtained on 14 September.
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' green beans were seeded at 65 pounds/acre on 9 May with four rows per plot on 30-inch centers. Triple superphosphate (130 pounds/acre) was banded at planting two inches beneath and two inches to the side of the seed row. The first 40 pounds N/acre was broadcast at planting; the remaining N was broadcast on 6 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 25 July. Plants were mowed to simulate machine-harvest and minimize further N uptake.
Beets and Carrots
The plot area received a broadcast, incorporated application of EPTC at 2.0 pounds/acre (beets) and 150 pounds triple superphosphate/acre. Carrot plots were treated with linuron at 1.25 pound/acre one day after seeding. 'Detroit Dark Red' table beet and 'Orlando Gold' carrot were seeded on 5 May with three rows on a five-foot bed. The first 40 pounds N/acre was broadcast on the planting date; the remaining N was broadcast on 10 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 25 July (beets) and 16 August (carrots). The remaining beet plants were pulled to minimize further N uptake.
Results and Discussion
Green beans responded with a yield increase to the first increment of applied N, but yield did not increase further with rates of N application greater than 40 pounds/acre (Table 29). This is somewhat unusual, as most experience with this crop indicates increasing yield up to 100 pounds applied N/acre. Carrot yield responded as expected, with the greatest yield at 120 pounds N/acre (Table 29). This is consistent with results previously obtained at NWREC. 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 30). This is also consistent with our previous research and with grower experience.
Soil nitrate concentration after bean harvest increased approximately linearly with increasing rate of applied N at the 1 and 2 foot depths, but the effect was significant only in the surface foot of soil. Soil ammonium content also increased with increasing applied N for the surface foot of soil (Table 31).
Soil ammonium concentration did not differ with rate of applied N for beet and carrot, but in each case soil nitrate concentration increased at higher rates of applied N for the first two feet of the soil profile (Tables 32 and 33). For carrot, there was no increase in soil nitrate content in the surface foot of soil until the applied N reached the optimal level for yield (Table 33). In the beet plots, the greatest soil nitrate content in the surface two feet of soil was with 180, rather than 240, pounds applied N/acre (Table 32). The same trend, although not significant, was apparent in the residual soil ammonium content in the beet plots. Further research will be needed to determine whether this observation was simply due to chance.
Table 29. Effect of rate of urea-nitrogen on the yield of green beans and carrots, NWREC, 1994 N rate Bean Carrot (lb/A) (T/A) (T/A) 0 4.5 15.8 40 7.5 21.0 80 6.8 23.1 120 6.7 26.0 160 7.3 24.0 Significance L*Q* L*Q* L=linear, Q=quadratic, *significant, p=0.05. Table 30. Effect of rate of urea-nitrogen on the yield of table beets, NWREC, 1994 N rate Yield (lb/A) (T/A) 0 3.7 60 13.6 120 16.4 180 17.6 240 18.0 Significance L**Q* Table 31. Effect of rate of broadcast nitrogen on soil nitrate and ammonium concentrations (ppm) at four depths following bean harvest, NWREC, 1994 N rate, lb/A Sample depth (inches) 0 40 80 120 160 LSD(.05) Pre-plant --------Post-harvest-------- Nitrate 0-12 0.7 1.6 7.7 10.2 9.9 16.4 14.1 12-24 0.7 1.0 2.7 3.9 3.7 5.3 NSD 24-36 1.6 1.3 2.3 5.5 3.4 3.8 NSD 36-48 2.3 1.3 2.3 5.0 3.9 3.1 NSD Ammonium 0-12 1.9 3.4 4.8 3.9 6.0 7.0 2.0 12-24 1.9 2.7 2.3 3.1 2.9 3.2 NSD 24-36 2.0 2.9 1.7 4.9 2.6 2.7 NSD 36-48 1.8 3.0 1.7 4.0 3.6 2.7 NSD Table 32. Effect of rate of broadcast nitrogen on soil nitrate and ammonium concentrations (ppm) at four depths following beet harvest, NWREC, 1994 Sample depth (inches) N rate, lb/A 0 60 120 180 240 LSD(0.05) Pre-plant --------Post-harvest------- Nitrate 0-12 1.5 0.8 1.4 3.0 6.6 3.9 1.7 12-24 1.2 0.3 0.6 1.0 1.9 1.6 0.6 24-36 1.1 0.9 1.2 2.0 1.6 1.9 NSD 36-48 1.0 1.9 2.9 3.3 2.2 4.4 1.9 Ammonium 0-12 0.8 2.6 3.0 5.3 6.1 4.1 NSD 12-24 2.7 3.1 2.5 3.2 5.5 3.2 NSD 24-36 3.7 3.2 2.4 3.6 4.2 2.0 NSD 36-48 3.3 3.3 2.4 3.8 2.7 2.0 NSD Table 33. Effect of rate of broadcast nitrogen on soil nitrate and ammonium concentrations (ppm) at four depths following carrot harvest, NWREC, 1994 N rate, lb/A Sample depth (inches) 0 40 80 120 160 LSD(.05) Pre-plant --------Post-harvest-------- Nitrate 0-12 1.5 0.8 0.4 0.9 5.5 8.2 6.6 12-24 1.2 0.4 0.5 1.6 1.9 1.5 1.0 24-36 1.1 1.6 1.7 3.8 3.8 2.8 NSD 36-48 1.0 2.1 2.9 3.7 3.2 3.0 NSD Ammonium 0-12 0.8 2.3 3.2 2.8 2.7 2.4 NSD 12-24 2.7 2.6 3.5 2.3 2.9 2.4 NSD 24-36 3.7 2.5 3.5 2.0 3.9 2.8 NSD 36-48 3.4 2.7 4.9 2.4 2.6 2.6 NSD