Iron chlorosis (yellowing) in oaks and other plants is usually due to lack of iron uptake rather than low iron content in the soil. The most common reason is high pH, although soil compaction may be involved as well. One option, of course, is to grow only plants adapted to high pH soils. Short of that, do what you can to amend the soil to incorporate more organic matter and lower the pH. Elemental sulfur is the most effective compound to reduce soil pH, but even it takes time to lower the pH.