When the leaves of a plant begin to turn yellow, determining the cause will take some investigation. There are several possibilities:
The first consideration is a lack of nitrogen, especially if a number of plants are affected and there has been no recent application of fertilizer. Apply a high nitrogen fertilizer, and if after a week or so the plants begin to turn green, the problem is solved.
If the plants don’t turn green, the problem could be a disease. Plants can become infected with a fungus, especially when over watered, causing a root or crown rot. Inspect the roots and the crown of the trunk. Scratch into the bark of the trunk at the soil line and also into a piece of root. If the tissue inside is brown or black, rather than light colored, a root or crown rot is evident.
Another common reason for a general yellowing is a lack of oxygen in the root zone. This is due to too much water – over irrigation or too much rain. As long as the soil drains well the problem is minimized. But in clay soils, water does not drain well, resulting in a waterlogged condition. Without adequate oxygen in the soil, roots cannot function properly, and the plant turns yellow. If the condition continues, root rot will develop and the plant will die. That is why so many planting instructions suggest to plant in a well-drained soil.
If plants are yellow and lack vigor, a soil analysis can identify nutrient deficiencies and/or a pH imbalance. The pH reveals the acidity or alkalinity of the soil. Most plants have a definite acid/alkaline range in which they will properly grow.
Although not common, there is always the possibility of persistent chemicals in the soil. A soil sterilant or preemergent herbicide may have been applied to the soil. These compounds can persist for many years. Preemergent herbicides are those that are sprayed on the bare ground and become incorporated into the top few inches of soil. At label dosages they affect only the target weeds and last from several months to a year. But if an overdose is applied, the chemical will be there for many years and adversely affect many plants.
An infestation of microscopic round worms called nematodes (example, root knot nematode) can also cause plants to yellow and be stunted.