If an occasional lower leaf yellows and falls, don't worry. This is normal. If several bottom leaves yellow at the same time and drop, the plant is getting too much water. Or the plant could be in a cold draft. Should leaves at the top yellow but remain firm, the cause could be the use of hard water.
City water can contain a lot of calcium, which ties up the iron in the soil.
When a plant cannot absorb needed iron, as well as magnesium, the result is yellowish leaves. Flush the soil occasionally to wash away unwanted buildups. Fertilize your plant biweekly or monthly during warm months. Iron chelate, applied according to package directions, will boost iron supplies and in turn help green up your plant. When you water, make sure it drains well.