It's time to come clean on potting soils. Beginning with the 1999 certification cycle, growers who apply for organic certification will be denied if they use potting mixes that contain synthetic fertilizers or wetting agents.

In general, this means that organic growers should avoid using most commercially available potting mixes. If in doubt about whether it is acceptable, error on the side of caution and whip up your own mix from scratch using ingredients you are familiar with.

A good potting mix provides water and air, retains fertilizer and supports the plants. It should have solid particles and a balance of small pores(to hold water) and large pores (to hold air). Not enough large pores will cause the mix to be heavy and promote disease. Too many large pores will make it hard to keep properly hydrated.

Potting mix ingredients will change depending upon how they are to be used. Seed starting mixes, for instance should be very light and full of air because seedlings die easily in heavy, wet soil.

To growers who are mixing their own, ATTRA (Appropriate Technology Transfer for Rural Areas) makes these suggestions:

• Sift all ingredients through a screen.
• Make sure limestone is well-mixed throughout the soil.
• Use only fully matured compost.
• When sand is called for, use sharp or builder's sand.

Wet the mix a day before using it, especially if it contains peat moss (which is hard to wet).

Finally, 'Finding the right blend of ingredients takes practice. Each producer must develop a recipe that works best for the type and size of plants grown.