The yellowing of houseplant leaves is a common issue caused by a variety of maintenance and pest problems. Maintenance issues including pot size and watering cause the yellowing of leaves in most cases. However, plant pathogens transmitted through the air, left over in the pot or found in the soil can cause diseases that cause the yellowing of leaves. Identifying the cause early is important for treatment and the overall health of your plant.
Fertilizer consists of three primary macro-nutrients: nitrogen, phosphorus and potassium. Nitrogen promotes cell division and plant growth, phosphorus helps plants with photosynthesis and provides energy for growth, while potassium helps plants absorb the other two macro-nutrients. According to NASA, plants grown in soil with a lack of nitrogen have thin stems and yellowish-green leaves. To avoid a nitrogen deficiency, use nitrogen-based potting soil formulated for the individual plant's needs. Nitrogen promotes leafy green vegetation; however, adding too much nitrogen to a houseplant can result in weak foliage growth. Recommended nitrogen levels vary from plant to plant, therefore it is important to gauge fertilizer application on an individual basis.
Over-watering houseplants leads to a variety of plant-related diseases and health hazards. Potted plants often have poor soil drainage, which causes moisture to build up in the soil from regular watering. According to the University of Minnesota, over-watering is a leading cause of fungal growth that causes the plant disease known as root rot. Root rot infects the root system of the plant, causing plant death within one to two weeks after infection. Early symptoms include yellowing of the leaves, mushy or rotted roots and leaf drop. The University of Minnesota recommends using well-drained porous soil to prevent over-watering and root rot. Over-watering also causes plants to suffocate, which is another leading cause of leaf yellowing.
According to the North Dakota State University, plants that outgrow their pots will exhibit yellowing of leaves or leaf death. Overgrown roots in potted plants eventually reach the bottom of the pot and cannot continue to expand, thus causing plants to stop growing and eventually to die. Another common symptom is leaf drop, which is the result of over-potted plants. Re-potting plants with larger pots and nitrogen rich soil stokes plant growth and the eventual return of green leafy foliage.