Issue: October 22, 2005
Homemade compost to homemade potting soil
I tried to use my homemade compost as potting soil. Some plants did okay but others died. What's wrong?
Homemade compost can be a useful ingredient in potting soil, but for most plants it cannot be used without adding other materials. There may also be plant disease organisms in the compost. If the plant materials put into the composting process were diseased, those disease organisms may have survived and must be dealt with.
Compost alone can become waterlogged and suffocate the roots of many plants. While some plants may be able to grow in the pure compost, many others will benefit from the addition of coarse sand, aquarium gravel, perlite, or other things to increase the porosity of the potting soil. Since the compost is pure organic matter, it may also have an acid pH which is harmful to some plants. Adding lime can help if this is the case.
The problem of plant disease organisms can be managed by pasteurizing the potting soil once the sand or other material is added. Pasteurization is accomplished by heating the moist potting soil to a temperature of 140 degrees F. for 30 minutes. At this temperature, insects, plant bacteria, and plant viruses are killed. Heating to 180 degrees F. may be necessary to kill most weed seeds. The higher temperatures may cause the accumulation of some toxic materials in the soil and should be avoided if possible.
The soil may be heated in an electric oven, over charcoal in a barbecue, or in a microwave oven. An added benefit of using the microwave oven is that the microwaves may kill weed seeds and won’t dry the soil. In an oven or over the charcoal, the soil should be enclosed in material to prevent excessive drying (in a baking pan covered with foil, for example).
Pasteurizing the soil should be done outside because the odors generated when the soil is heated are not pleasant. The implements used in pasteurizing (thermometer to monitor temperatures and containers) should not be returned to the kitchen but kept for use only in soil pasteurization. The oven or microwave may be used again in the kitchen after the odors have been dispersed.
By increasing drainage and assuring that the homemade potting soil made from homemade compost is disease free, you should have more success.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!