Monday, February 16, 2015

Onions, Pungent or Mild

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.