On the other end of the scale, and another Secret of the Green Thumb, is soil dryness:
YELLOW LEAVES MEAN TOO LITTLE WATER
When whole leaves, usually the older ones, turn yellow, paper-bag tan, and wilt, this is almost always a sign that the soil is too dry. And again, as with brown tips, yellow leaves can indicate a host of other problems, beginning with too much water in the soil.
WATER, WATER EVERYWHERE, BUT NOT A DROP TO DRINK
This may seem very puzzling until you consider that constantly wet soil basically kills the roots; if the roots aren't functioning to suck up water for the leaves, the leaves aren't getting any water, and they behave the same as they would if the soil were too dry - in other words, they turn yellow.
Some of the other situations that can be signaled by yellow leaves are: humidity too high or low, too much light, too little light, mineral deficiency or toxicity, too much salt buildup in the soil, fungus, bacteria, bugs, and breakage.
A little confusing? You'll notice that the list of conditions indicated by brown tips and yellow leaves are almost the same. No wonder people become discouraged when they try to find out what's wrong with their plants. The operative concept here is 98% of the time, most of the time. If you test for soil moisture as we've discussed, and you think that too much or too little moisture might be the source of your problems, you change your watering, either by watering less/more, or less often/more often. If the moisture level seems on target, then you can begin to investigate other possibilities, which we'll be talking about as we go along.
(by Marlie Graves)