Answer: Before you do anything drastic, look again at the yellowing leaves. If they're mostly yellow with green veins, there's a probability the surrounding soil is deficient in iron; if they are just yellow and discouraged-looking, they may need a little magnesium. If you have had the problem before that suggests a chemical imbalance. The market abounds with iron tonics, if that's what you (or rather your plants) need. The magnesium deficiency can be cured quite easily with a tablespoonful of Epsom Salts in a gallon of water.
There's always a possibility some other adverse condition is harming the plants, but the first line of attack is to improve their diet.
Q&A Library Searching Tips