Fertilizer trials with overwintered onions at the North Willamette Station indicated a strong yield increase with application of lime, an increase with gypsum (calcium sulfate), and higher yields with ammonium sulfate rather than other N sources. The yield response to gypsum and ammonium sulfate indicated that when soil pH, N, P, and K are optimal, S may be the limiting element in onion production.
A trial on spring-planted onions in 1984 indicated increased bulb size with liming, but no yield increase, because of reduced stands at higher pH. This stand reduction was in contrast to previous results with overwintered onions which indicated improved stands at higher pH, over the pH range of 5.0 to 6.2. The mean bulb weight of the spring-planted onions increased with gypsum when calcium nitrate was the N source, but not when ammonium sulfate was the N source, indicating a response to the sulfate in the gypsum.
The following trial was undertaken to further evaluate the effects of lime and gypsum on onion stands and yield, independent of N source.
Methods
Lime was broadcast and disked into a Willamette silt loam at 0 or 3 tons/acre in March, 1982, with four replications of each treatment in randomized block design. Soil pH in spring of 1984 averaged 5.5 and 6.0, respectively. On April 10, 1985, 120 pounds N/acre as urea was broadcast and incorporated into the surface 2 inches of soil. On April 11, the field was seeded with 'Sure Crop' onion with 3 rows/bed on 20-inch spacing between rows. Propachlor herbicide was applied at 4 pounds/acre immediately after planting and again on June 6 and July 12. Plots were also hand-weeded twice. Stand counts were made on May 6 and again on May 13. Methomyl and diazinon (0.5 pounds/acre of each) were applied on August 16 for thrip control.
The lime variable main plots were randomly split by application of gypsum at 0 or 150 pounds/acre on May 10. Each gypsum treatment was replicated twice per lime main plot. Another 60 pounds N/acre was applied to all. plots on June 4. Leaf samples were taken for tissue analysis on July 23 and soil samples for pH determination on August 6. Plants were topped and bulbs harvested on September 26 after separating into the size grades: No. 1, over 3.5-inch diameter; No. 2, over 2.3-inch diameter; No. 3, others.
Results
Soil pH was not measured just before planting, but soil samples taken in August indicated a drop in soil pH of about 0.5 units since the previous measurement in 1984. The difference between treatments was maintained, however (Table 1). The reduced pH probably reflects application of heavy rates of acid-forming N fertilizers following a dry winter in which normal leaching may not have occurred. In contrast to the previous year, onion stands were not affected by liming (Table 1).
There were no significant lime x gypsum interaction affecting any component of yield or leaf tissue mineral content. Only main effects are reported in Tables 2, 3, and 4.
Yield of jumbo (No. 1) onions was low, possibly limited by suboptimal N or by a heavy infestation of thrips. Thrip damage appeared to be less severe on gypsum-treated plots, but no damage ratings were made. Liming increased yield of No. 1 and No. 2 onions markedly, without reducing the yield of No. 3 onions (Table 2). This is because of the smaller number but greater size of No. 3 onions produced on limed soil (Table 3). Liming increased total yield by 28 percent and more than doubled the percentage of No. 1 plus No. 2 bulbs.
Gypsum application also increased yield, primarily by increasing the yield of No. 2 bulbs (Table 2). Mean bulb weight of No. 1 and No. 2 bulbs tended to be increased by gypsum, even though stands at harvest were slightly increased with gypsum. Mean weight of No. 3 bulbs and all bulbs was increased by gypsum application, and the percentage of No. 1 + No. 2 bulbs was increased 35 percent with gypsum. The highest percentage of No. 1 + No. 2 bulbs (44.9 percent) was obtained with the combination of lime plus gypsum.
Lime application decreased leaf N, Zn, Mn, and Mg concentrations and tended to increase leaf P and Ca concentrations (Table 4). Gypsum slightly increased leaf P concentration and increased leaf S concentration.
This trial confirms the positive effect of lime on mean bulb weight found in 1984. The desirability of providing an S source is indicated by the yield increase with gypsum. This response to an S source has not been found in all experiments, however.
Table 1. Effect of lime on soil pH and stand of spring-planted onions, 1985 Lime rate Soil pH Seedling stand (No./foot) (tons/acre) Spring 1984 Summer 1985 May 6 May 13 0 5.5 5.0 4.8 5.6 3 6.0 5.5 4.6 5.2 NS NS NS: no significant differences Table 2. Main effects of lime and gypsum on yield of spring-planted onions, 1985 Lime rate Yield (hundredweight/acre) Total bulbs (tons/acre) No. 1 No. 2 No.1+No. 2 No. 3 Total harvested/foot 0 0.0 63.3 63.3 127.8 191.1 3.8 3 6.2 135.2 141.4 125.1 266.5 3.8 *
Z
** ** NS ** NS +Gypsum 3.7 119.2 122.9 136.6 259.5 4.0 -Gypsum 2.5 79.3 81.8 116.3 198.1 3.7 NS * * NS * NS
Z
*, **, NS: significant differences between means at 5% level, and 1% level, and no significant differences, respectively Table 3. Main effects of lime and gypsum on mean bulb weight and grade distribution by number, 1985 Lime rate Mean bulb weight(g) Percent by number No. 1 No. 2 No. 1+ No. 2 No. 3 All No.1 No. 1+No. 2 0 -- 158 158 72 86 0.0 17.5 3 306 170 172 92 126 1.4 38.7 NS
Z
* * ** ** * ** +Gypsum 309 167 169 90 117 0.8 33.1 -Gypsum 303 161 162 74 95 0.6 23.0 NS NS NS ** ** NS *
Z
*, **, NS: significant differences between means at 5% level and 1% level, and no significant differences, respectively. Table 4. Main effects of lime and gypsum on leaf tissue mineral concentrations, 1985 Lime rate % ppm (tons/acre) N P K Ca Mg S Zn Mn Cu 0 4.23 0.314 4.6 1.77 0.37 0.30 28 232 9 3 4.04 0.326 4.1 1.84 0.32 0.35 23 101 9 *
Z
NS NS NS ** NS * ** NS +Gypsum 4.12 0.313 4.3 1.82 0.34 0.39 24 160 9 -Gypsum 4.15 0.326 4.4 1.79 0.35 0.27 26 173 9 NS * NS NS NS ** NS NS NS
Z
*, **, NS: significant differences between means at 5% level and 1% level, and no significant differences, respectively.