A plant may be healthy for years and suddenly the leaves start to turn yellow. Something has caused a change, and you need to play detective to find the cause and then follow through with a solution.
Too much or too little water will cause plant leaves to yellow. A quick assessment is to pick up the potted plant. If the plant is heavy, it has been overwatered and the roots cannot absorb the excess. A plant that is light is most likely lacking in water and is struggling to survive. Check the soil around outside plants for wet or dry conditions. Water the plant when the soil is too dry. If the plant is in soil that does not drain well, remove the plant and place it in a more suitable area with good drainage.
Yellow leaves may be the result of a lack of nitrogen or iron. First apply a fertilizer with nitrogen and check the results after three or four days. Stop using the nitrogen if there is no change in the yellow leaves and apply a fertilizer with iron. Fertilizers for houseplants and outside plants are available at garden centers. Read the label on the bag or container to see if your particular plant will benefit from the fertilizer.
Changing the location of a houseplant may affect its growth. A new room could have a different level of humidity that could affect the plant. Reducing or adding sunlight will also affect the plant. A cold draft from a window or exposure to an outside door might also be a problem.
Examine the soil and plant leaves for insect infestation. If insects have invaded the plant, you will need to identify the invader and purchase the correct powder or spray to rid the plant of the insect. A soapy solution of dish detergent and water may eliminate some bugs from houseplants, followed by wiping the bugs from the leaves.