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. 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 causes 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, 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. I am not recommending ending the application of organic matter, I just mention this to note there may be a limit to the amount of OM applied when growing onions.
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 at bulbing is 90°F compared to 50°F. This increase, though, may be
due to a greater uptake of sulfur because of the higher temperatures. Pungency also rises under dry growing
conditions.
