Yellow means caution, even in the garden. While leaf yellowing, chlorosis, may be a signal that there is a problem that requires attention, it may also be normal. Chlorosis is the scientific word used to indicate the full or partial yellowing of plant leaves or stems and simply means that chlorophyll is breaking down.
Normal Chlorosis – Yellowing leaves at the base of an otherwise healthy plant is normal; the plant is simply utilizing the nitrogen and magnesium for exposed leaves near its top rather than older, lower leaves.
Chlorotic Response to Light – Moving a plant from full sun to shade, or visa-versa, can cause yellowing leaves. Make sure that you grow and maintain your plant in the proper light.
Chlorotic Response to Moisture – Sudden changes in soil moisture may damage or kill plant roots. Most otherwise healthy plants, however, are able to grow new roots as they readjust. Maintain correct soil moisture or move the plants to a more favorable environment.
Mineral Deficiency – A shortage of some key mineral nutrients will cause chlorosis in plants. Often, a yellow leaf indicates a lack of nitrogen, however, magnesium, iron, sulfur or manganese deficiencies are indicated by yellowing leaves with prominent green leaf veination. A magnesium deficiency will manifest itself in the yellowing of older leaves. On the other hand, an iron deficiency presents itself in the yellowing of new or young leaves. A simple soil analysis will let you know what minerals or trace elements your soil is deficient in.
Soil Factors – Although the essential and trace elements may be present in the soil, many other factors affect how the plant uses and absorbs them. If the soil pH is too high/low or there is too much salt in the soil, the plant will not be able to utilize the available nutrients. Test your soil pH and adjust as necessary.
Toxins – Although this doesn’t happen frequently, pollutants like paint, oil, chemical solvents, airborne herbicides or pesticides or other pollutants may cause leaves to turn yellow and dark brown before dying. In this case, remove and dispose of the plant and its surrounding soil.