Mineral density of onion bulbs as affected by fertilizers based on elemental sulfur

The yield increase of vegetables, including common onion (Alium cepa L.), is of great interest to growers. However, higher yield often leads to what is known as genetic dilution of mineral density. A technology of onion fertilization based on elemental sulfur (S0) fertilizers seems to be a simple agronomic measure preventing a decrease in the nutrient concentration in onion bulbs. To verify this hypothesis, field studies were conducted in the 2009 and 2010 seasons. A two-factorial trial consisted of five sulfur fertilizers: Sw (crude form of S0), Sm (micronized S0), SmCu (Sm enriched with copper 0.25%), SmZn (Sm enriched with zinc 0.5%), and AS (ammonium sulfate); and, as the second factor, of two doses of S: 30 and 60 kg ha-1. The NPK plot, included as an independent experimental variant, was used as the control. Yield of onion increased by 13% in response to 30 kg S ha-1, and by 44% when fertilized with 60 kg S ha-1. The yield of bulbs was significantly affected by increasing magnesium and negatively by sodium concentration. The increase in both bulb yields and dry matter content resulted in a simultaneous decrease in nutrient density, except sulfur. The strongest dilution effect was observed for sodium (-33%), iron (-19%), magnesium (-17%) and phosphorus (-16%). Such a negative development can be prevented by applying sulfur fertilizers in an appropriate form. The concentrations of N, P, K, N, and Cu responded most demonstrably to the added ammonium sulfate. The concentrations of S, Zn, Cu, Mn, and Fe were affected by the micronized form of S0 enriched with zinc. The magnesium concentration was the highest in plants fertilized with the crude form of S0.