Chlorophyll
Chlorosis is a condition in which leaves do not make enough chlorophyll, so they become yellow. Chlorosis can be caused by a not having enough iron—called iron chlorosis—or by not having enough magnesium or nitrogen. Soil pH sometimes affects these kinds of chlorosis. Many plants are adapted to grow in soils with specific pH levels and their ability to absorb nutrients from the soil can depend on this. Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.