QUESTION: I grow a lot of flowers and vegetables in containers in my Port Hueneme garden because of limited ground planting space. Is there any problem with reusing commercial planter soil for successive plantings? By the way, I find the whole gardening section of the Times interesting and helpful!
ANSWER: The successful reuse of potting soil depends upon what the soil is to be used for, says Jim Downer, Ventura County Agriculture agent. Seed germination requires sterile soil, but epiphyllum orchids can tolerate just about any soil problem. Some bedding plants, like petunias, are particularly susceptible to fungi and are best grown in sterile soil, but other bedding plants don't require such fastidiousness.
To sterilize his potting soil, Downer heats it in his portable microwave, which he moves outside because the process can be smelly. He moistens the soil, places it in plastic containers with lids, and heats it for 5 to 20 minutes. The soil is sterile when its intended temperature reaches 200 degrees, tested with a meat thermometer inserted after the soil is removed from the oven. He keeps the containers covered until the soil cools.