Q: I live in a condominium, so I don't have much of a garden other than what I grow in containers. I use store-bought planter mix, but I'm perplexed by the wide variety of prices for what looks like the same volume of mix. Can you explain it to me?--T.M., La Habra
A: You cannot assume that one bagged planter mix is the same as another, whatever its purpose, or that any one in particular contains everything that every plant needs. No rules or laws apply to planter mixes or their labeling, and it's a rare label that shows an analysis of the mix's ingredients. Recent tests show that nearly all prepared planter mixes are permeable and provide good drainage, but the similarities stop there. Some mixes are deficient in phosphorus and therefore cause stunting. Others have pH balances tending toward one direction or the other, and their use in certain situations is sometimes detrimental. Planter mixes with unknown qualities should be used on a small scale on a trial basis. Try a quick test using rapid-sprouting radish seed to uncover problems.