Sulfur is an essential element required by onions. But high levels
of sulfur in the soil can contribute to pungency, that is, a strong flavor. The
compound responsible for this is called allyl propyl disulfide.
Furthermore, the term sweet, when referring to onions, is a
misnomer. Mild is the correct
description for onions with low pungency.
Growing
onions in high sulfur soils can give them a pungent flavor. In Central America, "sweet"
onions for export are not grown on soils that have sulfur levels above 18 ppm
(parts per million). Yet, for the average gardener, it is not realistic, or even necessary, to treat the
soil in order to lower the sulfur content.
In most soils, sulfates move well in the soil profile. Since onions are
a shallow rooted crop, it is easy for the sulfate to move below the root zone
of onions.
What is practical is to
eliminate or limit the application of sulfur fertilizers. Definitely do not
apply them after bulbs begin to form. As long as high levels of sulfur are not
applied, pungency should not be an issue.
Constant applications of
large amounts of organic matter (OM), especially manures, can potentially
increase the sulfur content of the soil. Though with ample rainfall, excesses
should be leeched beyond the onion roots.
In addition to sulfur in the soil, pungency or the strong flavor in onions can also develop when growing at high temperatures. Twice as much pungency may be expected when the temperature is 90°F compared to 50°F at the time the bulbs form. This increase may be due to a greater uptake of sulfur because of the higher temperatures. Pungency also rises under dry growing conditions.
Attention Big Island Residents: I will be teaching a class, Vegetables in the Home Garden, on Saturday, February 21, 9:30AM -12:30PM at UH-Hilo Campus. Call 974-7664 to register; there is a fee.
