Research report from OSU's North Willamette Agricultural Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture, NWREC
Introduction
Storage onion production in western Oregon has been almost exclusively on drained lake bottom soils which are high in organic matter. More recently, production of onions on mineral or "upland' soils has increased rapidly. Response of onions to nitrogen and potassium rates on mineral soils is not well understood. A trial in 1985 indicated that maximum yield and largest bulb size occur at 150 to 225 pounds N/acre on a Willamette silt loam. Onion yields did not respond favorably to K application, but some growers believe that high rates of K increase storage quality.
In 1986, yields of 'Granada' onion were greatest at 150 pounds N/acre while yields of 'Simcoe' were greatest at 250 pounds N/acre, on Willamette soil. In a grower field, 'Granada' yield did not vary significantly with N rates from 144 to 264 pounds/acre. In both experiments, rots and sprouting in storage were not affected by N rates. The 1987 and 1988 trials evaluated onion yield and keeping quality as a function of N and K rates.
Methods
After disking and harrowing, 500 pounds/acre of ammonium sulfate was applied to a Willamette silt loam, pH 5.9. Main plots of 30 x 15 feet were established by application of potassium chloride at 0, 100, or 200 pounds K2O/acre with four replications of each treatment. The KC1 was incorporated to a depth of four inches by rotary tillage and 5-foot-wide, 30-foot-long beds were seeded with three rows of 'Granada' onion with 20 inches between rows on 27 April, 1987. Propachlor herbicide was applied at 4 pounds/acre immediately after seeding, and was reapplied on 3 June and 2 July. Methomyl was applied at 0.45 pounds/acre for thrips control on 2 July.
Nitrogen rates of 0, 75, and 150 pounds N/acre (total N = 100, 175, 250 pounds/acre, respectively) as ammonium nitrate were applied to 5- x 30-foot subplots on 11 June. Plants were topped on 21 September and harvested on 23 September. Bulbs were size-graded into large (over 3inch diameter) and small sizes. The large bulbs were collected for a storage trial and rated for basal and neck rots and sprouting on 25 February, 1988.
Methods were similar in 1988. Seeding date was 15 April. The propachlor was reapplied once on 20 May along with oxyfluorfen at 0.25 pounds/acre. The methomyl application was on 20 June. Nitrogen rate subplots were established on 11 June and harvest was on 12 October. The 1988 storage trial ended on 14 February, 1989.
Results
Neither N nor K rate greatly affected onion yield or bulb size in 1987. Yield and bulb weight tended to be greatest at 100 pounds N/acre (Table 1). Mean bulb weight and percentage of large bulbs were significantly reduced at 250 pounds N/acre. Bulb size and yield also tended to be reduced at the highest rate of potassium application, perhaps due to the chloride provided by the KC1. As in 1986, the absence of significant rainfall during the April-June establishment period may have prevented leaching of nitrogen below the onion root zone.
The interaction of N and K rates significantly affected total yield (Table 2). Increasing the N rate decreased yield at the 0 and 100 pound rates of K20 but increased yield at the high rate of K.
Neither N nor K rate consistently affected rots of the neck or basal plate after storage (Table 1). Increasing N rate significantly increased the degree of sprouting; increasing K tended to reduce sprouting (Table 1). The sprout inhibition effect at high rates of K was particularly pronounced at high rates of N (Table 3).
In 1988, there were no significant interactions of N and K rates on yield; only main effects are presented in Table 4. The greatest yield of large bulbs (over 3 inches) occurred with 175 pounds N/acre. Total yield and mean bulb weight also tended to be greatest at this N rate, but the differences were not significant (Table 4). The highest percentage of large bulbs occurred with 175 or 250 pounds N/acre. These results are in contrast to 1987, when onion yields were greatest at 100 pounds N/acre, but are in agreement with results obtained in 1985 and 1986.
Also in contrast to results of the previous experiments, application of a low rate of K tended to increase yield of large bulbs and mean bulb weight. However, K application decreased total yield and the high rate of K also decreased the yield and percentage of large bulbs. Although not statistically significant, the high rates of K and N tended to reduce stands slightly. Part of the yield effect of the fertilizers may be due to their effects on stands and competition among plants.
Increased K tended to reduce sprouting in 1988 but the effect was not significant. Application of K reduced basal plate rots. Rate of N had no effect on rot or sprouting. The results of four years of fertilizer trials indicate that N applications should be moderate, in the range of 150 to 200 pounds/acre. Potassium application, even where soil test does not indicate a likely yield response, may lead to improved storage life.
Table 1. Main effects of N and K rates on yield and size of 'Granada' onion, 1987 Yield (50 lb bags/acre) Mean bulb wt. (g) % large % rot after storage % sprouting Treatment Large bulbs All bulbs Large All bulbs Basal Neck in storage N (lb/acre) 100 399 869 342 229 30.8 0.9 4.1 0.0 175 385 844 338 232 30.7 0.0 4.7 0.5 250 298 735 334 196 21.6 3.8 5.2 4.8 Significance NSz NS NS * * NS NS ** K2O (lb/acre) 0 381 844 339 226 29.7 0.0 6.1 2.9 100 385 823 331 224 31.4 2.4 6.1 2.4 200 298 781 343 206 22.3 2.3 6.3 0.0 Significance NS NS NS NS NS NS NS NS zNS, *, **: no significant differences, significant at the 5% and 1% levels, respectively. Table 2. Interaction of N and K rates on total yield (bags/acre) of onions produced, 1987 N rate K2O rate (lb/acre) (lb/acre) 0 100 200 100 962 1003 643 175 799 928 885 250 772 538 895 LSD (0.05) = 289 bags/acre. Interaction significant at P = 0.02. Table 3. Interaction of N and K rates on onion sprouting in storage, 1987 N rate K2O rate (lb/acre) (lb/acre) 0 100 200 100 0.0 0.0 0.0 175 0.9 0.6 0.0 250 7.9 6.6 0.0 Rated on 2/25/88. LSD (0.05) = 5.0%. Table 4. Main effects of N and K rates on yield and size of 'Granada' onion, 1988 Bulb yield Mean bulb % large % rot after % sprouting (50 lb bags/acre) wt. (g) bulbs storage in storage Treatment Large All Basal Neck N (lb/acre) 100 136 783 164 9.3 0.7 1.0 2.0 175 238 885 186 15.1 0.3 1.0 2.0 250 211 817 185 15.3 0.3 1.3 3.7 Significance * NS NS * NS NS NS K2 (lb/acre) 0 204 881 176 13.0 1.3 0.7 3.7 100 253 819 186 17.7 0.0 1.0 2.7 200 128 785 172 9.0 0.0 1.7 1.3 Significance * NS NS * * NS NS