Overwatering or Poor Drainage
Too much water can cause root rot and create nutrient deficiencies in the soil, either of which can cause leaves to yellow.
In well-drained soil, over-watering washes minerals and micro-nutrients out of the soil, meaning the plant can not get the nutrients to create or store energy it needs to maintain photosynthesis and growth.
In pots with poor drainage, plants roots that regularly sit in water or waterlogged soil will begin to decay, impairing the plant's ability to absorb needed water and nutrients.
When the leaves have just yellowed, you can reverse the problem by scaling back on water, removing the damaged leaves or ensuring good drainage. When a plant's stems are yellowing and dying, root rot has set in and the plant is unlikely to be salvageable.
Most plants require 6 to 8 hours of sunlight per day. This sunlight can be in the form of direct, indirect or artificial light. While plant light requirements vary widely among species, a sure sign of insufficient light is fading and yellowing of foliage. This indicates the photosynthesis process is not producing enough energy to fulfill the plant's needs. The deep green color of plant foliage comes from the chloroplast cells that carry out photosynthesis. Less sunlight equals fewer and more 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 a burning of the plant roots, throw soil nutrients out of balance and build the levels of nutrients in the soil beyond what a plant can use.
This overabundance 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 of excess fertilizer or transplanting into fresh potting soil 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 and soil nutrients. Without sufficient soil around the roots, water and fertilizer can wash right through the soil, preventing the plant from absorbing those nutrients. Without those nutrients, the yellowing of foliage will occur.
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.