This is an excerpt from the KSU Extension services – see the above link for more info.
Soil treatment. Lowering the soil pH is a more permanent way of correcting iron chlorosis caused by high pH soils. Unfortunately this is also one of the most difficult treatment methods because it is almost impossible to lower the soil pH significantly to depths of 18 to 24 inches.
A more feasible approach is to add iron to the soil in sufficient quantity to increase the amount available to tree roots. In bare soils, iron sulfate can be applied to the ground under the tree canopy at a rate of one pound per half inch of trunk diameter measured 4½ feet above the ground. Trees growing in turf-covered soil should be deep-fed either by liquid injection with a hydraulic sprayer or by iron sulfate placed in holes drilled into the soil.
Drill holes spaced two feet apart and 15 to 18 inches deep should form concentric circles around the tree, beginning two to three feet from the trunk and extending beyond the ends of the branches about three feet.. This treatment remains effective for two to three years. Chelated iron may be used, but be sure to follow label directions for rates. Best results for the current year will occur when treatment is made in early spring just as buds begin to swell. Also, high pH soils may prevent most iron chelates from working. For any soil with a pH above 7.2, use an iron chelate with EDDHA. Such products include Sequestar 6%, Sprint 138 and Millers FerriPlus.
I will use the method above, and make sure to use the chelate with EDDHA.
An alternate soil treatment is to mix sulfur and iron sulfate at a 1:1 ratio. The sulfur lowers the pH in a small area and makes the iron more available to the tree. The recommended rate is one pound of iron sulfate to one pound of sulfur per inch of trunk diameter at 4½ feet above the ground for trees four inches and under. The recommended rate for trees over four inches is two pounds of iron sulfate and sulfur mixture per inch of trunk diameter at 4½ feet above the ground.
Calculating how much iron I will need – diameter of tree is = 23″ x 2 = 46lbs of iron sulfate? How the heck am I going to get that much into the ground?
BONIDE IRON SULFATE – comes in 4lb boxes.