February 18, 2016 – Kenosha News
Question: I attended your recent class at the Southwest Library. You mentioned pasteurizing homemade potting soil. I’ve decided to make plan my own potting soil mix for seed starting this spring. Please give me directions on how to pasteurize my soil mix. Also, would you share your recipe again for sustainable potting soil mix that doesn’t use peat moss? M.M.
Answer: Although not mandatory for homemade potting mixes, pasteurization is a good practice because the process kills pathogens (disease causing fungi and bacteria), insects, and many weed seeds without killing beneficial soil organisms.
Pasteurizing potting soil at home is easy but stinky! Don’t invite your neighbors over for dinner the night after you pasteurize potting soil, unless you don’t want them to come back.
Place your potting soil in a roaster or kettle and cover with the lid or foil. The soil mix should be moist but not wet. Monitor the temperature with a food thermometer in the center of the potting soil, similar to the way you use a meat thermometer in a roast. Put the roaster of soil into a 200 to 250 degree Fahrenheit oven. When the thermometer reaches a temperature of 140 degrees Fahrenheit, start timing. After 30 minutes at 140 degrees it is “done”. Some sources recommend temperatures of 170 to 180 to kill weed seeds. If weeds are not a concern, keep the temperature at 140.
Do not let the potting mix get too hot. Higher than needed temperatures may destroy soil structure, kill good organisms and, in some cases, release toxins.
Some references suggest using a potato, instead of a thermometer, to determine doneness. You bury a small raw un-peeled potato in the center of the potting mix. Supposedly when the potato is cooked, the potting soil is pasteurized. This method seems a little chancy to me so I strongly suggest you stick with the thermometer.
Don’t forget to clean all your seed starting materials or use new ones. Often the source of a disease, like damping off, is contaminated old pots, flats or trays. Wash everything, including counter tops, in a ten percent chlorine bleach solution (1 part bleach and 9 parts water). Rinse well with plain water after washing.
Potting soils for seed starting must have good drainage to help prevent rotting and damping off disease. A potting soil comprised of 1/3 mature screened compost (worm compost, coir or shredded bark may be substituted for compost), 1/3 soil, and 1/3 sand may be used in place of a peat based mix.
Barb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at firstname.lastname@example.org or call 262-857-1942.