In a soil consisting mostly of limestone, it may not be feasible to reduce the p H of the soil, adequately. Fe is the answer. A chelating agent is a chemical which attaches to an Fe atom and prevents its becoming unavailable in the soil. Abundant organic matter in the soil can serve as a natural source of chelation. But you can also purchase chelated iron.
Different brands vary widely in price and effectiveness.
They may work at around p H 7, but won't be effective in a soil with higher p H levels. Sequestrene Fe 330 and Versene are examples of this type of chelator. As might be expected, the higher the p H at which a material will be effective is, the more the product is likely to cost. In most soils, controlling the p H is better in the long-run than merely correcting deficiency symptoms with a chelator. It is seen first on the oldest leaves on the plant, but in severe cases, may also affect the newer leaves.