Leaves start to turn yellow when a plant either receives too much or too little water. Pest infestation, nutrient deficiency or high PH in the soil can also cause leaves to yellow. In some cases, leaves turn yellow owing to chlorosis, a condition where leaves are devoid of chlorophyll.
Yellow leaves are also an indication that a plant is receiving too much or too little sunlight. This occurs mostly with indoor plants.
In some cases, the yellowing of leaves is a natural process. For example, in autumn, as the nights grow longer and the temperatures cool, the leaves of some deciduous trees turn yellow and begin to shed.