Overwatering or Poor Drainage
Too much water for your plants is not a good thing. Over-watering causes root rot and nutrient deficiencies in the soil as the constant watering is washing the minerals and micro-nutrients out of the soil. Over-watering can also occur due to poor drainage of your pot so that the plants roots are regularly sitting in water or waterlogged soil. When the leaves have just yellowed, scaling back on water, removing the damaged leaves and ensuring good drainage will likely reverse the trend. When a plants stems are yellowing and dying root rot has probably set in and the plant is not likely to be salvageable.
Most plants require 6 to 8 hours per day of sunlight to remain healthy. This sunlight can be in the form of direct, indirect or artificial light. Plant light requirements vary widely among species but a sure sign of insufficient light is fading and yellowing of foliage. This indicates that there is not enough light to run the photosynthesis process in which the plant produces its own food. The deep green color of plant foliage comes from the chloroplast cells that carry out photosynthesis. Less sunlight equals fewer and anemic chloroplast cells which translates into lightening color. In essence the plant is starving to death and cannot support its leaves so they are yellowing and dying.
Overuse of fertilizers can lead to burn of the plant roots, imbalanced soil nutrients as well as build up of nutrients in the soil beyond levels a plant can use. This over abundance of minerals and micro-nutrients can stress the plant, force weak top growth and disrupt its nutritional cycle. These conditions can lead to yellowing or plant foliage. Scaling back feedings, watering to rid the soil or excess fertilizer or transplanting into fresh potting soil can typically bring the plant back to life.
Root Bound, Underwatered & Underfed
When the root system of a plant crowds out the surrounding soil in the pot the roots lose easy access to water or dissolved minerals. Water and fertilizer applied to a root bound pot often wash right through at watering time with no soil to hold them in place for a time. This can lead to yellowing of foliage. The situation can often be corrected with transplanting to a larger pot with an adequate amount of fresh nutrient rich potting soil and good drainage.