Storage onions in western Oregon have been grown almost exclusively on lake bottom soils which are high in organic matter (more than 10%). Recently, production of onions on mineral or "upland" soils with low organic content and N availability has increased rapidly and now equals production on the organic soils. Response of onions to nitrogen rate and to applications of phosphorus and potassium on the mineral soils is not well understood. The 1985 trial evaluated onion yield, size grade, and keeping quality as a function of nitrogen rate, broadcast and banded application of P, and a K application. The 1986 trial evaluated onion yield and keeping quality as a function of nitrogen rate and phosphorus application.
Methods
1985
After disking and harrowing, 75 pounds N/acre as ammonium sulfate was applied to a Willamette silt loam, pH 6.0 on April 10, 1985. The field was then divided into plots of 5 x 30 feet and potassium chloride at 150 pounds K/acre and treble superphosphate at 60 pounds P/acre were applied to the appropriate plots and worked into the surface two inches of soil. Plots were then seeded with three rows of onion with 20 inches between rows. On April 11, two of the four replicates were seeded with 'Sure Crop'; the remaining replicates were seeded with 'Benny's Red', because of a shortage of 'Sure Crop' seed. Propachlor herbicide was applied at 4 pounds/acre immediately after planting and was reapplied on June 6 and July 12. Plots were also hand-weeded once.
The eight fertilizer treatments were as follows: 1) 75 pounds N/acre as ammonium sulfate (N1, no sidedressed N); 2) 75 pounds/acre additional N as ammonium nitrate (N2); 3) 150 pounds additional N/acre as ammonium nitrate (N3); 4) N2 plus 60 pounds P/acre banded 2 inches to the side and 2 inches beneath the seed row; 5) N2 plus banded P at 60 pounds P/acre plus broadcast P at 60 pounds/acre; 6) N3 plus banded P; 7) N3 plus banded P plus broadcast P; 8) N plus banded P plus 150 pounds K/acre, broadcast as potassium chloride. In addition, 225 pounds N as ammonium nitrate was applied to border plots (N4). All ammonium nitrate was sidedressed on June 4. Treatments were applied in randomized complete block design except for N4. Leaf samples were collected from all treatments on July 23. Plots were harvested after topping on September 24. Bulbs were separated into three size grades: No. 1, over 3.5-inch diameter; No. 2, over 2.3-inch diameter; No. 3, others.
1986, Experiments 1 and 2
Methods were as in 1985 except that 50 pounds N/acre as ammonium sulfate was applied to a Willamette silt loam on April 21, 1986. Five-fool wide, 30-foot long plots were seeded with three rows of 'Granada' onion ith 20 inches between rows on April 22. Concentrated superphosphate was banded two inches to the side and two inches beneath the seed row of appropriate plots. Propachlor herbicide was applied at 4 pounds/acre immediately after seeding and was reapplied on June 12, July 1, and July 29. Plots were also hand-weeded twice. Diazinon was applied at 20 pounds/acre on July 1, and azinphosmethyl and oxydemetonmethyl were applied at 0.75 pounds/acre on July 15 for thrips control. Azinphosmethyl was reapplied on July 29.
The eight fertilizer treatments included 100, 150, 200, and 250 pounds N/acre in factorial combination with banded P at 0 or 60 pounds P/acre. The additional N was sidedressed as ammonium nitrate on June 13. Treatments were in randomized block design with four replications. Plots were harvested after topping on October 7. Bulbs were separated into two size grades with the No. 1 grade at least 3.0 inches diameter, and the No. 2 grade consisting of all others. The No. 1 bulbs were collected for a storage trial. Bulbs were removed from storage and evaluated for rots and sprouting on January 21, 1987.
The methods for Experiment 2 were the same, except that the variety was 'Simcoe,' the seeding date was May 19, and onions were not kept for storage.
1986, Experiment 3
The trial was planted by grower Jim Schlecter on his field of Woodburn clay loam. Methods were Schlecter's standard practice except that N rates were established following seeding. The base fertilizer program included 103 pounds N/acre as ammonium sulfate, 205 pounds K20/acre as potassium chloride, 2.0 tons lime/acre, and 41 pounds N and 61 pounds P/acre injected beneath the seed row as 10-34-0 solution. The trial was seeded with 'Granada' onion, 4 rows/bed, on April 10. Additional N was sidedressed as urea on May 15. The eight treatments included N rates of 144, 184, 224, and 264 pounds N/acre in factorial combination with broadcast P at 0 or 107 pounds P/acre. Treatments were replicated four times in randomized complete block design.
Results
1985
Yield of No. 1 (jumbo) bulbs tended to increase up to 225 pounds N/acre (N3) and dropped off sharply at 300 pounds N (Table 1). At 150 pounds N/acre, banded P tended to increase No. 1 yield. Addition of P also tended to increase No. 1 yield at the 225-pound rate of N. At both rates of N, the combination of banded P plus broadcast P tended to yield more jumbo onions than did banded P alone. Addition of K did not increase yield of jumbos beyond that obtained with banded P alone.
Yield of No. 2 onions did not vary significantly with treatment, but there was a trend towards increasing yield of No. 2's with increasing N up to 225 pounds/acre. At 150 pounds N/acre, banded P tended to increase No. 2 yield.
Trends for yield of No. 1 plus No. 2 bulbs and total yield were similar. Yields were highest at 225 pounds N/acre and banded P increased yield at 150 pounds N/acre but not at 225 pounds/acre. Application of K did not increase yield.
Within size categories, N, P, and K had very little effect on mean bulb weights (Table 2). Mean bulb weight for all bulbs did increase with 150 rather than 75 pounds N/acre but P and K did not affect mean bulb weight of all bulbs.
The major effect of increasing N rate up to 225 pounds/acre was to shift bulbs from the No. 3 category into the No. 2 category (Table 3). The percentage of No. 1 bulbs and No. 1 plus No. 2 bulbs was highest at 150 pounds N/acre. The slight reduction in percentage of large bulbs at 225 pounds N/acre may be caused by the higher average stand on these plots (Table 3). Stand differences were not significant, but there was a trend toward an inverse relationship between stands and the percentage of large bulbs, Phosphorus and K did not have a consistent effect on the percentage of No. 1 or No. 1 plus No. 2 bulbs.
The fertilizer applications had little effect on leaf tissue elemental concentrations (Table 4). Potassium application increased leaf K content. Leaf P concentration tended to increase with P application and leaf N concentration tended to increase up to the 225 lb/acre rate of N application, but these differences were not significant. The high rate of N reduced leaf P content. Both leaf Zn and Mn concentrations tended to increase with K application.
Table 1. Effects of N, P, and K on yield of onions, 1985 Treatment No. 1 No. 2 No. 3 No. 1 + No. 2 Total ------------hundred weight/acre-------------- N1 56.5 343.1 157.9 399.6 557.5 N2 89.0 356.2 118.8 445.2 563.0 N3Z 92.1 496.0 151.9 588.1 740.0 N4 15.3 247.3 239.6 262.6 502.2 N2 P band 96.4 455.8 127.5 552.2 679.7 N2 P+P band 117.1 453.1 117.6 570.2 687.8 N3 P band 124.4 383.9 100.2 508.3 608.5 N3 P+P band 140.8 465.4 135.4 606.2 741.6 N3 K+P band 104.1 452.6 160.8 556.7 717.5 LSD(0.05) 61.8 NS NS 134.7 165.4 ZNot randomized with other treatments NS: no significant differences Table 2. Effects of N, P, and K on mean bulb weight of onions, 1985 Treatment No. 1 No. 2 No. 3 No. 1 + No. 2 Total --------------------g/bulb------------------- N1 318 204 96 214 159 N2 347 213 117 240 207 N3Z 321 203 99 215 175 N4 295 172 87 177 117 N2 P band 335 205 107 219 185 N2 P+P band 347 208 106 230 192 N3 P band 350 208 107 233 196 N3 P+P band 340 207 100 227 185 N3 K+P band 356 212 101 229 178 LSD(0.05) NS NS NS NS 40 ZNot ramdomized with other treatments NS: no significant differences Table 3. Effects of N, P, and K on number of bulbs harvested and grade distribution by number, 1985 Treatment No. bulbs/30 row feet No.1 No. 1 + No. 2 No.2 No. 3 -----------------%----------------- N1 189 5.5 53.6 48.1 46.4 N2 160 14.8 69.5 54.7 30.5 N3Z 222 6.9 64.6 57.7 35.4 N4 217 1.3 32.4 31.1 67.6 N2 P band 196 8.1 68.2 60.1 31.8 N2 P+P band 191 11.0 69.4 58.4 30.6 N3 P band 169 13.2 70.9 57.7 29.1 N3 P+P band 209 10.2 66.3 56.1 33.7 N3 K+P band 210 7.5 60.7 53.2 39.3 LSD (0.05) NS NS 12.1 8.6 8.6 ZNot randomized with other treatments NS: no significant differences Table 4. Effects of N, P, and K on onion leaf mineral content, 1985 Treatment N P K Ca Mg S Zn Mn Cu --------------%---------------- -----ppm------ N1 3.5 0.38 3.4 1.3 0.20 0.78 21 58 9 N2 3.7 0.38 3.6 1.4 0.22 0.78 22 57 9 N3Z 3.8 0.39 3.5 1.3 0.20 0.77 23 61 9 N4 3.8 0.30 3.7 1.4 0.22 0.64 23 109 8 N2 P band 3.8 0.39 3.5 1.3 0.20 0.83 20 51 9 N2 P+P band 3.7 0.43 3.2 1.1 0.18 0.87 21 46 9 N3 P band 3.8 0.40 3.4 1.3 0.20 0.78 21 49 9 N3 P+P band 3.8 0.41 3.9 1.3 0.19 0.78 21 54 9 N3 K+P band 3.9 0.41 6.2 1.3 0.20 0.79 24 73 9 LSD(0.05) NS NS 1.9 NS NS NS 3 16 NS ZNot randomized with other treatments NS: no significant differences
1986, Experiment 1
Banding P beneath and to the side of the seed reduced stands by nearly 12% (Table 5). The N rate had no effect on stands, as expected, since the N variable was not applied until the stand was established. Total yield decreased 7% with banded P, attributable to the decreased stand. Yield of No. I bulbs tended to increase with banded P but the increase was not statistically significant. Mean bulb weights were increased with banded P and the percentage of large bulbs tended to increase. Banding P slightly increased basal plate rots of onions in storage. Soil available P content averaged 134 ppm, a high level. The response of onions to banded P in the presence of high levels of available soil P is in agreement with results obtained for several other crops on Willamette soil, including sweet corn, bush beans, and overwintered onions.
Nitrogen rate did not significantly affect yield of large onions or total yield, but in each case the trend was for the highest yields to occur at 150 pounds N/acre. Mean bulb weight and percent large bulbs were also greatest at this N rate but mean weight of large bulbs was greatest at 200 pounds N/acre. In each case, the differences were not statistically significant. Nitrogen rate did not significantly affect rotting or sprouting of bulbs in storage.
1986, Experiment 2
Banded P again tended to reduce stands but the difference was not significant. Mean bulb weight again tended to increase with banded P (Table 6).
The percentage of large onions was very small in this trial, partly because of the late planting date, the slightly greater stands of 'Simcoe,' and the tendency of 'Simcoe' to form smaller bulbs than 'Granada.' Heavy incidence of mildew and blast also may have contributed to the small 'Simcoe' bulb size, only slightly greater than half the mean bulb size for 'Granada.'
Yield and bulb size tended to increase with increasing N rate, but the differences were not significant.
1986, Experiment 3
Broadcast P increased yield of No. 1 bulbs and tended to increase mean bulb weight and percent of No. 1 bulbs (Table 7). Total number of bulbs harvested/plot was not affected by the broadcast P. Nitrogen rate had no effect on any component of yield. In the absence of broadcast P, the percentage of large bulbs increased linearly with increasing N rate. In the presence of P, the percentage of large bulbs was greatest at the lowest N rate. Neither N rate nor P significantly affected rots of onions in storage. No sprouting was observed.
The lack of a response to N in 1986 differs from the strong yield increase up to 150 to 225 pounds N/acre seen in 1985. In the case of Experiment 3, the high base rate of N may have been optimal. In all experiments, the below normal rainfall during the growing season, particularly in June, may have prevented leaching of N from the root zone.
Table 5. Main effects of N rates and banded P on yield and size of 'Granada' onion, Experiment 1, 1986 Treatment Seedling Yield Mean bulb wt. No. 1 Rot after Sprouting stand/ No. 1 bulbs All bulbs No. 1 All bulbs storage in storage foot Basal Neck ---50-lb bags/acre---- -----(g)----- ---------------%-------------- N rate (lb/acre) 100 3.2 320 762 306 230 32.8 10.1 13.3 4.3 150 3.1 362 774 309 240 38.9 10.2 15.5 2.2 200 3.2 324 762 313 233 33.7 7.1 23.4 1.0 250 3.1 348 770 312 231 33.4 8.4 13.9 8.2 NSZ NS NS NS NS NS NS NS NS P rate (lb/acre) 0 3.3 310 796 303 220 29.7 5.1 16.0 3.5 60 2.9 368 740 317 247 39.7 12.8 17.1 4.4 * NS NS * * NS ** NS NS ZNS, *, **: no significant differences, significant at the 5% level and 1% level, respectively. Table 6. Main effects of N rates and banded P on yield of 'Simcoe' onion, Experiment 2, 1986 Treatment Seedling Yield Mean bulb No. 1 stand/foot (bags/acre) wt.(g) bulbs (%) N rate (pounds/acre) 100 4.0 612 125 0.1 150 3.9 612 133 0.7 200 3.5 644 134 0.3 250 3.6 658 141 0.6 NS NS NS NS P rate (pounds/acre) 0 3.9 632 130 0.2 60 3.7 630 137 0.8 NS NS NS NS Table 7. Effects of N rates and broadcast P on yield and size of 'Granada' onion. Experiment 3, 1986 N rate P rate Yield (50 lb bags/acre) Mean bulb wt. (g) Percent % rot after (lb/A) (lb/A) Total No. 1 bulbs No. 1 bulbs All bulbs No. 1 bulbs storage Basal Neck 144 0 1047 506 336 205 28.8 4.0 1.5 184 0 859 517 338 197 29.9 8.0 5.0 224 0 898 444 310 200 32.2 4.0 7.8 264 0 1037 563 336 208 34.0 4.0 2.0 144 107 1001 545 326 212 35.4 4.0 8.0 184 107 998 515 346 212 31.5 7.8 5.3 224 107 1058 563 334 211 33.5 0.0 5.3 264 107 934 506 324 198 32.8 0.0 2.0 LSD (0.05) NS 35 NS NS 5.1 NS NS Main effects, N rate 144 lb/A 1024 525 331 208 32.1 4.0 4.8 183 929 516 342 204 30.7 7.4 5.1 223 978 504 322 206 32.9 4.0 6.0 263 986 535 330 203 33.4 4.0 2.0 NS NS NS NS NS NS NS Main effects, P rate 0 lb/A 960 507 329 202 31.2 5.0 3.8 107 998 532 333 208 33.3 2.8 5.1 NS * NS NS NS NS NS Interaction NS NS NS NS * NS NS