There are almost as many formulas for lightweight potting mix as there are gardeners who need it. And light weight is important to those who worry about heavy planters on balconies and decks.

For starters, try this recipe, which offers both light weight (for the roots) and substance (for the plants).

Moisten each ingredient, then thoroughly combine: one part vermiculite, one part compost, two parts loam or topsoil, three parts coarse perlite and three parts filler (decayed bark, shredded coconut fiber, peat moss or any combination).

Expect to fertilize regularly and to replace the mixture every few years. All of the components except the soil will break down over time.

Perlite, in case you were wondering, is a heat-expanded mineral like vermiculite. But vermiculite is like a sponge, while perlite is like a sponge- covered rock, so it drains better.

Perlite is very close to inert, so it doesn't change soil pH. Vermiculite usually turns alkaline when exposed to constant moisture.