Clay pots probably need to be sterilized in an oven:
http://crittercreeklab.s5.com/ . I don't use clay pots for a number of reasons. The biggest of which is that when it is time to repot, the roots are so firmly attached to the clay, that damage has to be done to them to remove the plant. In plastic, one needs merely to flex the pot and the roots break free with no damage. Shards in the bottom of clay pots is an invitation for snails and slugs to have a happy home. Plastic pots have numerous smaller holes, which do a good job of draining and to a small extent venting. Clay is too heavy for my hardware cloth covered benches. I have plastic pots in azalea types every 1/2 in, and bulb pans every 1 ". They store easily by nesting, and are cheap enough to use a new one every time I change pot size for a plant. Hmmm, there may be more reasons, but can't think of them just now. Cynthia, Prescott, AZ