Apply in the spring or fall and repeat in fall or spring. This treatment may need to be applied for several years for the best results. If plants remain chlorotic, iron can be supplied to plants in different forms. The two principal types of iron containing materials are iron chelates, an organic form, and inorganic compounds in soluble form, such as ferrous sulfate. Iron chelates are marketed under various trade names and in various formulations.
Ferrous sulfate is less available and less effective than the chelates.
The iron in chelates remains available to plants when the chelates are placed in the soil while inorganic forms often become unavailable. Some formulations of iron chelate can be applied to the foliage, but this approach is usually not as permanent as soil applications. Follow the manufacturer's recommendations for application rate. Tree implants containing iron chelates are also available for treating chlorosis. Implantation eliminates the problem of spraying, requires less time and equipment than soil applications and is less troublesome.