It is not a good idea to fertilize a sick plant. It seems almost counterintuitive, but fertilizing stressed plants just adds to their stress. The best thing you can do is feed the soil, not your plant. If you are concerned that your soil is deficient in a nutrient, get your soil tested so you will learn what it is lacking. If you are seeing a few yellowing leaves, this is not a nutrient issue. An entire plant that is displaying yellowing leaves may or may not have a nutrient deficiency. We advise consulting your local garden center if your plant appears to be failing to get more specific advice.