Aloha Nick, I would like to grow sweet slicing onions on my property of Puna. My lot is pahoehoe (lava), ripped and leveled with about 6 inches of cinder. In my beds I add a cinder/soil mix, mac nut compost, and top it with composted greenwaste mulch. I will plant a short day granex onion. Is there any way to build a bed with low-sulphur soil? Or any way to treat my beds to remove some sulphur? My research suggests the sweetest onions are grown in low-sulphur soil. I am after a “mild” slicing onion better than “sweet.” The sugar content may be high even in a pungent onion. I just want to grow some onions that don’t have much bite so they can be used raw in salads and sandwiches. Do you have any seed recommendations? Mahalo, D.
Let me first discuss sulfur in the soil. 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.
You are correct; 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.
For the short day length seed varieties, the University of Hawaii recommends: Granex33, Yellow Granex hybrid, Mercedes, Rio Bravo, Rio Zorro, Cougar, Sweet Sunrise, Jaguar, Monsoon and Savannah Sweet.
Here are some basic tips from Master Gardener and Master Herbalist Laureen Campbell on using herbs.
Drying: Herbs that are not fully dried are extremely susceptible to mold. In East Hawaii it is difficult to completely dry herbs unless a dehydrator is used. Never use herbs that are molding.
Storing: Once dried, place herbs in glass containers with sealer lids.
Fresh: To get the most out of fresh herbs, chop them finely and add raw to the top of soups, stew, salads etc. Raw herbs contain more of the medicinal and healthful properties than cooked.
Making infused cooking oils or vinegars: Chop fresh herbs and cover completely with the oil or vinegar. Check daily, shake and make sure the herb remains below the level of the liquid.
Storage for infusions and dried herbs: Store in a dark, cool place. Light degrades both oils and the herbs.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at firstname.lastname@example.org.