In the N source trial, the number of bulbs harvested per plot varied significantly with treatment, but the stands may have varied before the treatments were applied (Table 4). Total yield varied with N source, but the differences were not directly proportional to stand differences. Mean bulb weight and percent No. 1 bulbs were greatest with ammonium sulfate, in spite of a greater than average stand. They were lowest with ammonium chloride, in spite of a low stand. The high percentage of No. 1 bulbs and high mean bulb weight with ammonium sulfate confirms the efficacy of this fertilizer observed in 1982/83.
Among the other fertilizers, there was no clear advantage for ammonium-N over nitrate-N sources. Adding the sulfur source, gypsum, to ammonium nitrate did not improve yields. Although the amount of S provided by the gypsum (36 pounds/acre) was much lower than that provided by the three ammonium sulfate applications (171 pounds/acre), it should have been sufficient to cause a yield response if S were deficient in the soil. Gypsum at this rate also did not increase yields in a parallel study of the effects of lime, P, and gypsum on overwintered onions in 1984/85. However, in a study of the effects of lime and gypsum on spring-seeded onions in 1985, this rate of gypsum increased yields significantly.
Source of N had no effect on leaf tissue concentrations of N, P, K, Ca, Mg, Zn, and Cu (Table 5). Leaf Mn concentration was highest with the acid-forming ammonium chloride and ammonium sulfate. These levels of Mn are all in the normal range and should not have affected yield. Leaf S concentration was highest with ammonium sulfate or with ammonium nitrate plus gypsum, indicating plant availability and uptake of the fertilizer sulfate. Although leaf S levels were low with the other N sources, there was no correlations between leaf S levels and yield, because of the low yields with ammonium nitrate plus gypsum.
While confirming that ammonium sulfate is a good spring N source for overwintered onions, this trial provided no new information on the relative importance of the ammonium or sulfate ions in providing the yield response. The increase in leaf Mn with ammonium sulfate, and tendency toward increased leaf Zn content with this fertilizer, may indicate that ammonium sulfate, through its acidifying effect on the soil, is increasing availability of micronutrients which were limited in availability on this well-limed soil.
Table 18. Effect of N source on yield of overwintered onions, 1985 Total bulbs No. 1 bulbs Total No. 1 Mean wt. Mean wt. Percent harvested/plot harvested/plot yield yield All bulbs No. 2 bulbs No. 1s --------No./24 feet----------- --tons/acre-- -------ounces---------- Amm. nit. 54 12 10.9 4.6 5.9 10.4 23 Amm. sul. 68 19 15.0 6.7 6.5 10.0 28 Cal. nit. 59 15 12.2 5.3 6.2 10.4 26 Amm. chl. 55 3 7.4 1.1 3.9 9.7 6 Urea 74 15 12.8 4.8 5.1 9.5 19 Amm. nit.+gypsum 56 11 11.1 3.8 5.8 10.0 20 LDS (0.05) 10 3 4.0 2.8 1.4 NS 14 Table 19. Effect of N source on onion leaf tissue elemental concentrations, 1985 N Source N P K Ca Mg S Zn Mn Cu ----------------%---------------- -----ppm------ Amm. nitrate 3.9 0.20 2.34 0.82 0.13 0.22 21 47 4.6 Amm. sulfate 3.5 0.19 2.25 0.80 0.12 0.37 21 61 4.3 Cal. nitrate 4.0 0.22 2.39 0.82 0.13 0.21 19 47 4.1 Amm. chloride 3.3 0.20 2.23 0.82 0.12 0.18 20 86 4.1 Urea 3.4 0.19 2.17 0.88 0.12 0.22 18 51 3.9 Amm. nit.+gypsum 3.7 0.19 2.15 0.76 0.12 0.34 18 41 4.3 LSD(0.05) NS NS NS NS NS 0.05 NS 8 NS
