December 31, 2007

Heat Key to Pasteurizing Soil

By Garden Variety CURTIS SMITH For the Journal

Q: I want to use my own garden soil and homemade compost for my flower and vegetable garden transplants this year. How deep should I make a tray of soil to be pasteurized in the barbecue?

The New Mexico State University Web site mentioned heating the soil to 140 degrees for a half-hour to kill bacteria, viruses and insects or 180 degrees to include weed seed.

What happens to the soil's beneficial insects and mycorrhizae fungi? -- R.B.

A: When pasteurizing soil for transplants, temperature and duration are the critical elements to consider. The depth of the container alters the time needed for the heat to reach the center of the soil. Use a thermometer to test the temperature in the potting soil.

A meat thermometer will work. But once it has been in soil, never use it with food.

Years ago a friend said she would put a small Irish potato in the center of the soil. When the potato was cooked, her soil was pasteurized. She then composted the potato. This is less accurate, but it will work.

Using the outdoor barbecue for pasteurizing soil keeps the unpleasant odors outside. But the soil should be moist, and not soggy. It shouldn't be put over the hottest part of the fire, as that would cause some soil to overheat before the center of the soil reached the proper temperature. Overheating and heating without sufficient moisture will cause salts to accumulate, which may damage the seedlings.

Overheating also may cause the soil to be sterilized. When pasteurizing soil, some beneficial organisms remain and quickly colonize the soil when it cools. The presence of the beneficial organisms helps reduce the re-entry of plant-disease organisms. If the soil is sterilized, the plant pathogens may more easily recolonize it.

Many gardeners find it easier to purchase potting soil from a garden store. These commercial soils have been pasteurized before shipping and should have no plant pathogens unless the package was ripped. Even if the package had been opened, the presence of the beneficial organisms should reduce the level of pathogenic organisms.

Some beneficial organisms survive pasteurization, but insects, their eggs, many bacteria and fungi don't.

The beneficial fungi mycorrhizae that grows inside and outside a plant's roots helps a plant absorb water and nutrients. The fungi can be reintroduced by taking some unpasteurized soil from around a plant and mixing the soils.

Garden calendar

Saturdays and Sundays, 9 a.m., Bird Walks at the Rio Grande Nature Center. 2901 Candelaria NW. Call 344-7240.

Sundays, 1 p.m., Nature Walks at Rio Grande Nature Center. Call 344-7240.

Jan. 12, 2 p.m., "The Importance of Plants in a Well-Designed Xeriscape" by David Salman of Santa Fe Greenhouses and High Country Gardens. $10 at Santa Fe Greenhouses, 2904 Rufina St. in Santa Fe. Call (505) 473-2700 or (800) 811-2700.

Albuquerque Master Gardeners will accept applications until Monday. for the 2008 Master Gardener program beginning Jan. 8. Applications are at, or at the Extension Office, 1510 Menaul NW, 243-1386. Contact Jane Cole, or Jo Anderson at [email protected]

Send gardening questions to Garden Variety, Attn: Dr. Curtis Smith,

NMSU Cooperative Extension Service, 9301 Indian School NE, Suite

112, Albuquerque, NM 87112.

Phone, 275-2576; fax, 292-9815;

e-mail [email protected]

(c) 2007 Albuquerque Journal. Provided by ProQuest Information and Learning. All rights Reserved.