Ferrous sulfate plays an important role in chlorophyll development in plants. Chlorophyll allows plants to process light and make food. Insufficient ferrous means that the plant isn't eating enough. Ferrous is a natural component of soil, but under certain conditions, even though the ferrous is present, the plants are unable to absorb it.
Ferrous deficiencies are most common in alkaline soils. Ferrous deficiency is also common, however, in areas of new development where the topsoil has been removed. It is common in the desert and areas of the Southwest where calcium carbonate builds up in the soil. In alkaline soils, there is usually enough ferrous, but the soil's high pH -- 6.5 and above -- makes it unavailable to plants.
The most common symptom of ferrous deficiency is a yellowing of the foliage. The leaf veins stay green. Because the plant isn't getting enough food it will also appear stunted and sickly. Symptoms often appear at the top of the plant, and then move downwards gradually.
It can be difficult and time-consuming to counteract the effects of ferrous deficiency. There are ferrous compounds such as ferrous sulfate monohydrate and ferrous sulfate heptahydrate that can be applied directly to the foliage of plants to green them up quickly, but these need to be applied repeatedly in order to have any significant impact on the plant.