Fertilizer trials with overwinter onions at the North Willamette Station have shown a strong onion yield response to application of lime and gypsum and, in one experiment, a higher yield with ammonium sulfate rather than ammonium nitrate as N source. The yield response to gypsum and ammonium sulfate indicates that when soil pH, P, K, and N are optimal, S may be the element most limiting to onion bulb development. The following trial was designed to investigate the effects of lime, gypsum, and form of N on spring-seeded onions.
Methods
The lime variable main plots were formed in 1981 by application of 0 or 3 tons limestone flour/acre to 30 foot x 120 foot plots of Willamette silt loam. Lime plots were in randomized block design with four replications of each treatment. Resulting soil pH in spring 1984 averaged 5.5 and 6.0, respectively. 'Benny's Red' onion was seeded in 3-row beds, 20 inches between rows, approximately 8 seeds/foot, on May 14, 1984. Propachlor herbicide was applied at 4 pounds/acre on May 15 and was reapplied at the same rate on June 11 and July 13. Plots were also hand-hoed several times.
The lime main plots were split on June 13, 1984, by a sidedressed application of factorial combination of gypsum (CaSO4) at 0 or 150 pounds/acre and calcium nitrate or ammonium sulfate at I00 pounds N/acre. The N treatments were reapplied at the same rate on July 10. Individual subplots were one bed x 30 feet. Plants were topped two inches above the bulb and all plots were harvested on October 16, 1984.
Results and Discussion
Liming significantly reduced the original plant stand (Table 1). This is in contrast to many previous stand establishment experiments at the North Willamette Station, in which application of lime has generally increased stands of several small-seeded vegetables, including onions, over a pH range of 5.0 to 6.4. Stands of overwinter onions have tended to increase with increasing soil pH in previous experiments. An explanation for the stand decrease with liming in this experiment is not apparent, but the effect was consistent over all replications of the main plots and almost every bed, regardless of subplot treatment. The post emergence sidedress application of gypsum and N had no effect on stands (data not shown).
The stand difference was maintained through harvest, with a significantly higher number of bulbs harvested from unlimed plots (Table 1). Gypsum and N source did not significantly affect the number of bulbs harvested/plot. Neither lime, gypsum, nor N source significantly affected gross yield on an area basis.
Liming significantly increased mean bulb weight. Since there were fewer onions on limed plots, the increase in bulb size could be caused by reduced competition for water or nutrients. However, because of generally poor stands, the highest bulb density for any subplot was only 3.2/foot (1.9/square foot), indicating that competition among plants or crowding was not a factor in bulb size. Liming may have increased availability of P or Ca or reduced toxicity due to Al, Mn, or other metals.
Gypsum and N source did not significantly affect bulb weight in this experiment, although there was a trend toward larger bulbs with gypsum and calcium nitrate. Highest mean bulb weight was obtained with the combination of lime, gypsum, and calcium nitrate (Table 2). Gypsum increased mean bulb weight with calcium nitrate but not with ammonium sulfate as N source, giving some evidence for a yield response to sulfate-S.
Further experiments are planned to investigate the apparent stand reduction on limed soil and the magnitude of the onion response to a sulfate source.
Table 1. Main effects of lime, gypsum, and N source on spring-seeded onion stand and yield, 1984 Stand # bulbs Yield Mean bulb Treatment (seedlings/ft) harvested/foot (T/A) wt. (oz) Limed 1.7 1.7 9.7 6.9 Unlimed 2.2 2.2 9.8 5.4 **Z * NS * +Gypsum -- 2.0 9.9 6.4 -Gypsum -- 2.0 9.6 5.9 NS NS NS Calcium nitrate -- 1.9 9.7 6.4 Ammonium sulfate -- 2.1 9.8 5.9 NS NS NS Z**,*,NS: significant at 1% and 5% levels, and non-significant, respectively. Table 2. Interaction of lime, gypsum, and N source on spring-seeded onion mean bulb weight, 1984 Lime +Gypsum -Gypsum (T/A) Calcium nitrate Ammonium sulfate Calcium nitrate Ammonium sulfate ---------------------------------oz--------------------------------- 0 6.1 5.3 4.7 5.5 3 7.8 6.3 7.0 6.3 Interaction LSD (0.05) = 2.4