What makes the foliage of some of my plants turn yellow?

If your plants are yellowing, they may not be getting enough nitrogen from the soil. You may be overwatering or underwatering or giving them too much sun. Or your heat may be too high. African violet leaves may turn yellow if they are wet with cold water. Yellowing is also caused by insects or diseases. Check all the possibilities, because yellowing foliage is a sure sign of some trouble.