But don't give a plant too much nitrogen too quickly, or it will also cause stress and possibly continued chlorosis. Add organic compost, plant legumes, or use a balanced, slow- release fertilizer to steadily increase nitrogen in the soil. When a plant in your garden begins to turn yellow, and you cannot determine the cause by observing the environment or correct the problem by adding nutrients, then the only way to determine the cause is to test the soil. Some pollutant, like paint, oil, or a chemical solvent, may have been buried, spilled, or leached into the soil, or some airborne herbicide, pesticide, or other pollutant may be invading the plant through its leaves and stems. Soil testing will usually indicate the presence and possible severity of these toxins.
Toxins affect plants in several ways.
If they have killed the roots, the stems and leaves will turn yellow, usually at the growing tip. This chlorosis is soon followed by leaves turning dark and then the plant dies. While some toxins won't kill plant tissue directly, they hinder or prevent absorption of soil nutrients. Still other toxins act directly on plant proteins and chlorophyll to interrupt the photosynthetic process. While the plant may not die, it might flounder indefinitely and develop other problems.