Dear Khadijeh,

Foliar sprays of chelated iron or ferrous sulfate in the case of iron deficiency give a very quick but temporary greening of foliage. Chelated iron is water soluble form that is readily available for plant uptake at higher pH values and is generally more effective than sulfates in foliar sprays. Foliar sprays only affect treated leaves and not those produced later in the growing season. The resulting green-up of foliage usually occurs within one to two weeks but lasts at best one growing season; repeated applications will be necessary to maintain green foliage. Foliar sprays are most easily applied to shrubs and small trees and can be done by anyone with a handheld sprayer. Large trees are more easily treated with alternative methods. 

Chelated iron fertilizers provide iron in a soluble plant-available form in neutral and high pH soils. Annual application of these products incorporated into the top 1-2 inches of soil around plants in spring is another treatment for iron chlorosis. Another method that distributes fertilizers deeper into the root zone involves applying chelated iron uniformly across many 2" diameter holes drilled 12-15" deep into the soil using a soil auger. Starting 2-4 feet from the tree trunk, holes are placed 2-3' apart and cover the area under a tree canopy to a point just beyond the dripline of the tree. A third method of application is to dissolve chelated iron in water and inject it under pressure into the soil.

Three commonly used chelating agents are EDTA, DTPA, and EDDHA and they vary in their ability to provide iron over a wide range of soil pH. EDTA maintains iron in a soluble form up to a soil pH of only 6.3, DTPA is effective up to a pH of 7.5, and EDDHA maintains iron in a soluble form from pH 4.0 to 9.0. It is important to read product labels carefully and to know the pH of your soil in order to choose the right iron chelating product. These products are available in both solid and liquid forms at garden centers.

Soil applications of chelated manganese products are not recommended when manganese deficiencies occur from elevated soil pH. Chelates have a greater affinity for iron than manganese. When chelated manganese is applied, exchange of soil iron and the chelated manganese occurs, iron is taken up by plants instead of manganese, and the manganese deficiency in plants is intensified.

I agree with most of what has already been said. Maybe we should consider two additional points: (1) Cost and (2) stability vs. dissociation.

(1) EDDHA is much more expensive and hence for the farmer there is no point using EDDHA as long as EDTA would do the job just as well.

(2) We should bare in mind that any micronutrient that formed a complex with a synthetic chelator is not taken up as such, but the complex got to be split before uptake. Hence, the stability of the chelator just needs to be high enough to avoid release of the nutrient before reaching the plant (inner or outer) surface for uptake. Regarding soil it was corrected stated that EDDHA is more stable at high soil pH and high Ca concentration - hence it's the ideal chelator for calcareous soils. To the contrary, at leaf surfaces the Ca concentration is low and the pH usually not as high, and certainly not well buffered. Hence, what advantage would EDDHA deliver in such an environment?

In addition, as far as I recall 'Eisen-Sequesteren' (= Fe-EDDHA) offered by Bayer was recommended for use in grapevine orchards, but not exclusively for soil application.

Pore size on leaf surface is the main consideration for foliar applications of chelates. On an avg., pore size varied from 5-35 micrones. Therefore, sppecies with small pore size, chelates cannot be applied as foliar. May be in other species, theywill be effective.