Introduction
Vegetable growers in the Willamette Valley use high rates of nitrogen fertilizers, often exceeding 300 pounds actual N/acre per season. While growers claim 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 nitrogen may be contributing to nitrate pollution of groundwater. Improved efficiency of nitrogen management in vegetable crops may be possible if the fertilizer could be applied at the time of maximum crop need and placed for maximum contact with the root system. Sources of nitrogen may differ in efficiency of N uptake because of differences in volatility, degree of leaching, or crop preference for ammonium or nitrate.
A trial in 1991 investigated broccoli yield response to a wide range of rates of nitrogen as well as different methods of fertilizer placement. Method of placement did not affect yield. Rates of applied N up to 250 pounds per acre did not significantly increase the soil nitrate or ammonium concentration at the end of the growing season. Cauliflower is planted at lower populations and with greater between-row spacing than is the norm for broccoli. In a 1992 trial cauliflower yield increased with increasing rate of applied nitrogen up to 240 pounds/acre. Yield did not vary with band or broadcast placement of fertilizer at planting but there was a trend toward greater yield with the banded application. Broadcast application of sidedressed nitrogen was superior to banded application. The purpose of these trials was to confirm results obtained in 1992 and to investigate the effects of timing of application of sidedressed nitrogen as well as the form of the sidedressed nitrogen. A second purpose was to study soil nitrogen accumulation as a function of rate of applied nitrogen and presence of a winter catch crop and to monitor loss of nitrate through the soil profile in the winter following the crop.
Methods
'Snowball Y' cauliflower was direct-seeded in a Willamette silt loam, pH 6.0, at the NWREC on 10 June in both 1993 and 1994. Rows were 12 inches apart with a plant density of about two/inch in the row. Plot preparation included a broadcast and incorporated application of 10N-8.7P-16.7K fertilizer at 600 pounds/acre, boron at 2.0 pounds/acre, ammonium molybdate at 2 ounces/acre, trifluralin at 0.75 pounds/acre, and chlorpyrifos at 1.3 pounds/acre. On 27 July, 1993 (20 July, 1994) the seedlings were lifted with a shovel and transplanted bare-root into their final location. Soil preparation was the same as for the seed bed. Transplants were set in rows 2.5 feet apart with 18 inches between plants in the row. Plot size was three rows, 20 feet long. All three rows were harvested.
The initial application of 40 pounds N/acre was either broadcast as urea or banded as calcium-ammonium nitrate solution (CAN-17) three inches to the side of the row immediately after transplanting and irrigated in. The remaining nitrogen was broadcast or banded on the appropriate plots on 2 September or on 2 and 21 September, 1993 (24 August or 24 August and 21 September, 1994) depending on the treatment (Table 6). Sidedress N source was either urea, calcium nitrate (CN), or CAN-17 as appropriate for the treatment. Treatments were in randomized complete block design with four replications. The plots were sprinkler-irrigated as necessary and harvested on 8 and 19 October, and 2 November, 1993 (10 and 21 October, 1994). Two sets of plots were overseeded with 'Wheeler' cereal rye on 7 September to determine the feasibility of overseeding in cauliflower as a means of establishing a winter nitrogen catch crop. Following the last harvest in 1993, soil in treatments 1, 2, 3, 4, 5, and 11 was sampled to 48-inch depth in 12-inch increments. The plots were maintained through the winter in order to resample for soil nitrogen content and cover crop biomass accumulation. Shoots were clipped to 1 inch above the soil surface on 18 April. After weighing, subsamples were weighed, dried, reweighed, and submitted for analysis of total N. The soil of treatments 1-5 and 10 were sampled in 1-foot increments to 5-foot depth on 21 April. Samples were frozen and submitted for analysis of nitrate and ammonium content.
Results and Discussion
For the plots receiving only broadcast applications of urea, yield and mean head weight increased with increasing rate of nitrogen to a maximum at 240 pounds N/acre in 1993 and 180 pounds in 1994 (Table 7). The same trend was not true for the percentage of Grade No. 1 heads (defect-free), as only an increase from no applied nitrogen to the lowest rate of 60 pounds/acre caused an increase in quality in 1993 and N rate did not affect curd quality in 1994. In 1992, quality continued to increase with increasing increments of applied N. Previous work at the OSU vegetable farm suggested that the optimum rate of nitrogen application to cauliflower is in the range of 150 to 200 pounds/acre, similar to the results obtained in these trials.
Banded versus broadcast application of N at planting had no significant effect on yield or quality in 1993 (Table 8). In 1992, there was a greater trend toward higher yield and head size with a banded application but it was also not a statistically significant effect. Apparently, even with rows 30 inches apart, enough feeder roots establish in the soil between the rows that concentrating the fertilizer near the plant row is not a great advantage. This is in agreement with results obtained on broccoli grown on 16 or 20-inch row spacing.
Banded versus broadcast application of the sidedressed nitrogen also did not result in significant differences (Table 8). This is in contrast to 1992, when greater yield and mean head size occurred with a broadcast application. However, the single greatest yield in this trial was with the combination of broadcast fertilizer at planting, broadcast sidedress fertilizer, and a rate of 240 pounds N/acre.
Overseeding cereal rye about four weeks (1993) or seven weeks (1994) before first harvest did not reduce cauliflower yield (Table 9). This is consistent with results obtained in 1992, even though the rye development was greater than in 1993 and 1994 than in 1992.
Use of calcium nitrate, rather than urea, as the source for the sidedressed nitrogen had no effect on yield for the sum of the harvests in either year (Tables 10 and 11). Calcium-ammonium nitrate, as compared to urea, also had no effect on yield or head size in 1994 (Table 12).
Splitting the sidedressed N application such that half was not applied until eight or nine weeks after transplanting had no effect on yield or quality (Tables 11 and 13). However, there was a significant interaction of N source x timing of application affecting mean head weight and gross yield in 1994 (Table 11): splitting the sidedress N application decreased yield with urea as N source, but increased it with calcium nitrate. Further testing would be needed to determine if this effect is reproducible and biologically significant. Number of heads harvested at either harvest was not affected by N source or splitting the sidedress N application, indicating these factors did not affect maturity (data not shown).
In 1993, cauliflower effectively depleted the root zone of nitrate and ammonium at all rates of applied N except 240 pounds/acre (Table 14). Even at the high rate of N, residual soil nitrate was comparable to that before any fertilizer was applied. Apparently, cauliflower is similar to broccoli in N uptake efficiency, even though it was planted at a lower plant population and wider between-row spacing. Calcium nitrate versus urea as N source had no effect on residual soil nitrate and ammonium levels at harvest (Table 15). Splitting the sidedressed N also did not affect residual soil N concentrations (Table 16).
Cereal rye growing on plots that had been fertilized with 120 or 240 pounds N/acre accumulated a shoot dry weight of 0.55 and 1.19 tons/acre, respectively, on 18 April. This corresponds to nitrogen uptake of 16 and 36 pounds N/acre, respectively. Considering that only about 45 pounds of mineral N was present in the top foot of soil immediately after harvest of plots fertilized with 240 pounds N/acre, uptake of 36 pounds N/acre appears to represent recovery of a significant portion of the residual mineral N. Overseeding a grain into cauliflower may be a practical alternative for establishing an effective N catch crop in this late-harvested vegetable crop.
When re-sampled in the spring of 1994, after 24 inches of precipitation, rate of applied N had no effect on soil ammonium concentration (Table 17). Soil nitrate concentration tended to increase with greater rates of applied N. The nitrate concentration of the surface 10 inches of soil decreased from those immediately after harvest, while that of the next 30 inches of the profile increased, indicating probably of movement of nitrate with the winter rainfall. The rye cover crop significantly reduced soil nitrate concentration at all but the 20-30 inch depth, but had no effect on soil ammonium concentration.
Table 6. List of treatments, cauliflower N utilization trials, NWREC No. Total N Placement Placement and timing applied at planting of sidedress -------------------------------lb/A--------------------------------------------- 1993 1 0 0 None 2 60 40 broadcast, urea 20 broadcast, 5 weeks 3 120 40 broadcast, urea 80 broadcast, 5 weeks 4 180 40 broadcast, urea 140 broadcast, 5 weeks 5 240 40 broadcast, urea 200 broadcast, 5 weeks 6 120 40 banded, urea 80 broadcast, 5 weeks 7 120 40 broadcast, urea 80 banded, 5 weeks 8 120 40 banded, urea 80 banded, 5 weeks 9 120 40 broadcast, urea 80 broadcast, 5 weeks; overseed, 6 weeks 10 240 40 broadcast, urea 200 broadcast, 5 weeks; overseed, 6 weeks 11 120 40 broadcast, urea 80 broadcast as calcium nitrate, 5 weeks 12 240 40 broadcast, urea 200 broadcast as calcium nitrate, 5 weeks 13 120 40 broadcast, urea 40 broadcast, 5 weeks; 40 broadcast, 9 weeks 14 240 40 broadcast, urea 100 broadcast, 5 weeks; 100 broadcast, 9 weeks 1994 1 0 0 0 2 60 40 broadcast, urea 20 broadcast, 5 weeks, urea 3 120 40 broadcast, urea 80 broadcast, 5 weeks, urea 4 180 40 broadcast, urea 140 broadcast, 5 weeks, urea 5 240 40 broadcast, urea 200 broadcast, 5 weeks, urea 6 120 40 broadcast, urea 80 broadcast, overseed, urea 7 240 40 broadcast, urea 200 broadcast, overseed, urea 8 120 40 broadcast, urea 80 broadcast, calcium nitrate 9 180 40 broadcast, urea 140 broadcast, calcium nitrate 10 120 40 broadcast, urea 40 bcast, 5 weeks; 40 bcast 9 weeks, CN 11 180 40 broadcast, urea 70 bcast, 5 weeks; 70 bcast 9 weeks, CN 12 120 40 broadcast, urea 40 bcast, 5 weeks; 40 bcast 9 weeks, urea 13 180 40 broadcast, urea 70 bcast, 5 weeks; 70 bcast 9 weeks, urea 14 180 40 banded, CAN-17 140 banded, 5 weeks, CAN-17 Table 7. Effect of rate of broadcast urea nitrogen on yield, head size, and quality of cauliflower, NWREC N rate Mean head Grade No. 1 Total yield (lb/acre) wt. (g) heads (%) (tons/acre) 1993 0 565 39.5 7.1 60 664 62.5 8.5 120 809 62.7 10.6 180 978 58.6 11.9 240 1002 53.9 12.4 LSD (0.05) 170 16.6 2.6 1994 0 580 41.1 6.3 60 740 60.2 8.2 120 856 56.7 10.1 180 1046 66.5 12.3 240 1043 61.2 12.1 LSD (0.05) 162 NS 2.2 Table 8. Effect of broadcast versus banded application of initial and sidedressed nitrogen on yield, head size and quality of cauliflower, 1993 Placement Placement Mean head Grade No. 1 Total yield at planting at sidedress wt. (g) heads (%) (tons/acre) Broadcast Broadcast 809 62.7 10.6 Banded 924 60.2 11.3 Banded Broadcast 923 51.5 10.9 Banded 889 63.0 11.0 Broadcast at planting mean 867 61.4 11.0 Banded at planting mean 906 57.8 11.0 Significance, planting NS NS NS Broadcast at sidedress mean 866 57.1 10.8 Banded at sidedress mean 906 61.6 11.2 Significance, sidedress NS NS NS Table 9. Effect of overseeding cereal rye on cauliflower yield, head size, and quality at two rates of nitrogen, NWREC Treatment N rate Mean head Grade No. 1 Total yield (lb/acre) wt. (g) heads (%) (tons/acre) 1993 Overseeded 120 859 66.3 10.7 240 988 54.5 11.2 Mean 924 60.4 11.0 Not overseeded 120 809 62.7 10.6 240 1002 53.9 12.4 Mean 906 58.3 11.5 Significance NS NS NS 1994 Overseeded 120 898 60.7 10.4 240 1025 60.0 11.6 Mean 962 60.4 11.0 Not overseeded 120 856 56.7 10.1 240 1043 61.2 12.1 Mean 950 59.0 11.1 Significance NS NS NS Table 10. Effect of sidedressed nitrogen source on cauliflower yield, head size, and quality at two rates of nitrogen, NWREC, 1993 N source N rate Mean head Grade No. 1 Total yield (lb/acre) wt. (g) heads (%) (tons/acre) Urea 120 809 62.7 10.6 240 1002 53.9 12.4 Mean 906 58.3 11.5 Calcium nitrate 120 857 67.7 10.2 240 972 60.2 12.2 Mean 915 63.9 11.2 Significance NS NS NS Table 11. Effect of sidedressed nitrogen source and timing on cauliflower yield, head size, and quality at two rates of nitrogen, NWREC, 1994 N source Timing N rate Mean head Grade No. 1 Total yield (lb/acre) wt. (g) heads (%) (tons/acre) Urea early 120 856 56.7 10.1 180 1045 66.5 12.3 Mean, early 951 61.6 11.2 late 120 783 61.9 9.1 180 951 62.0 10.9 Mean, late 867 61.9 10.0 Mean, urea 909 60.0 10.6 Calcium nitrate early 120 849 61.5 9.6 180 966 64.2 11.1 Mean, early 908 62.9 10.4 late 120 904 65.5 10.4 180 1092 55.1 13.4 Mean, late 998 60.3 11.9 Mean, CN 953 61.6 11.1 Significance, N rate ** NS ** N source NS NS NS Timing NS NS NS Source x Timing ** NS ** Other interactions NS NS NS Table 12. Effect of urea versus CAN-17 as N source on cauliflower, head size, and quality at 180 pounds N/acre, NWREC, 1994 N source Mean head Grade No. 1 Total yield wt. (g) heads (%) (tons/acre) Urea 1045 66.5 12.3 CAN-17 1063 61.4 12.3 Significance NS NS NS Table 13. Effect of splitting the application of sidedressed nitrogen on cauliflower, head size, and quality at two rates of nitrogen, NWREC, 1993 Timing of Application N rate Mean head Grade No. 1 Total yield (lb/acre) wt. (g) heads (%) (tons/acre) All at 5 weeks 120 809 62.7 10.6 240 1002 53.9 12.4 Mean 906 58.3 11.5 Half at 5 weeks, 120 966 61.5 11.8 remainder at 8 weeks 240 928 67.5 11.8 Mean 947 63.5 11.8 Significance NS NS NS Table 14. Effect of rate of broadcast nitrogen on soil nitrate and ammonium concentrations (ppm) following final cauliflower harvest, 11 November, 1993 N rate, lb/A 0 60 120 160 240 LSD(.05) Depth of sample (inches) Pre-plant -----------Post-harvest------------ Nitrate 0-10 10.9 0.3 0.6 0.6 5.8 10.8 7.9 10-20 5.2 0.6 0.4 0.3 1.3 1.4 NSD 20-30 3.3 1.3 0.5 0.6 1.0 1.2 NSD 30-40 2.3 1.4 1.2 1.1 0.8 1.3 NSD Ammonium 0-10 5.1 2.0 2.3 2.5 3.6 5.3 2.0 10-20 4.9 1.3 1.7 1.8 2.4 2.2 NSD 20-30 3.8 1.7 1.8 2.0 1.8 1.8 NSD 30-40 3.5 1.7 1.6 1.8 1.5 1.5 NSD Table 15. Effect of nitrogen source and rate on soil nitrate and ammonium concentrations (ppm) following final cauliflower harvest, 11 November, 1993 Urea Calcium nitrate Depth of sample (inches) N rate, lb/A 120 240 Mean 120 240 Mean LSD (0.05)z Nitrate 0-10 0.6 10.8 5.7 0.6 7.9 4.3 7.9 10-20 0.3 1.4 0.8 0.4 1.4 0.9 NSD 20-30 0.6 1.2 0.9 0.9 1.2 1.0 NSD 30-40 1.1 1.3 1.2 1.0 1.3 1.2 NSD Ammonium 0-10 2.5 5.3 3.9 2.4 3.6 3.0 2.0 10-20 1.8 2.2 2.0 1.6 1.9 1.8 NSD 20-30 2.0 1.8 1.9 1.6 1.8 1.7 NSD 30-40 1.8 1.5 1.7 1.5 1.6 1.6 NSD zLSD for N source x N rate interaction. Main effect of N source nonsignificant for both ammonium and nitrate at all depths. Table 16. Effect of splitting the sidedress urea application and nitrogen rate on soil nitrate and ammonium concentrations (ppm) following final cauliflower harvest, NWREC, 11 November, 1993 Single sidedress Split sidedress N rate, lb/A Depth of sample (inches) 120 240 Mean 120 240 Mean LSD (0.05)z Nitrate 0-10 0.6 10.8 5.7 0.8 6.0 3.4 7.9 10-20 0.3 1.4 0.8 0.4 1.6 1.0 NSD 20-30 0.6 1.2 0.9 0.9 1.6 1.2 NSD 30-40 1.1 1.3 1.2 1.2 1.3 1.2 NSD Ammonium 0-10 2.5 5.3 3.9 2.2 6.1 4.1 2.0 10-20 1.8 2.2 2.0 2.0 1.9 2.0 NSD 20-30 2.0 1.8 1.9 1.9 1.9 1.9 NSD 30-40 1.8 1.5 1.7 1.6 1.7 1.6 NSD zLSD for sidedress x N rate interaction. Main effect of splitting the sidedress N application nonsignificant for both ammonium and nitrate at all depths. Table 17. Effect of rate of applied nitrogen and a rye cover crop on residual soil nitrate and ammonium concentrations, 28 April, 1994 Sample depth Rate of applied urea, lb/acre LSD (0.05) (inches) 0 60 120 180 240 240 (cover) ------------------------ppm--------------------- Nitrate 0-10 0.5 1.1 1.7 0.8 2.6 0.8 1.2 10-20 0.6 1.2 1.6 2.7 5.0 3.0 0.8 20-30 0.5 1.1 1.4 4.5 4.4 3.7 2.2 30-40 0.3 0.6 0.8 2.1 4.0 2.7 1.3 Ammonium 0-10 2.4 2.0 2.5 2.1 2.3 2.2 NS 10-20 2.6 2.1 2.3 1.9 2.0 1.9 NS 20-30 2.2 2.0 2.0 2.1 1.8 2.0 NS 30-40 2.0 1.5 1.7 1.7 2.6 1.7 NS