I was recently asked to give a consultation on the state of the plants in a private garden. I live and garden in central Israel, which has a typical Mediterranean climate, similar to places like southern California, south west Australia, and central Chile. I was inspecting the plants together with the homeowner, and while looking at a citrus tree, the lady suddenly said, "Oh and please show me how to apply iron fertilizer - I'm not very happy about having to use it every two weeks or so".
She had apparently been "advised" by a neighbor's gardener that the fruit trees should be so treated during the growing season. I looked again at the tree and what I saw were lovely dark green healthy leaves and plenty of decent looking fruit hanging on the branches. Her remark typifies to some extent, some of the misconceptions common amongst home gardeners on the important subject of plant nutrition.
Nutrient
Iron is one of the twelve minerals essential to plant growth and development. It is a micro-element, meaning that the plant needs it in relatively small quantities, as opposed to the macro-elements, Nitrogen, Phosphorous and Potassium, which are required and consumed in larger amounts. Iron is rarely absent from soils, at least in dry climates, but a high soil pH, meaning alkaline conditions, prevent it dissolving in the soil water, thereby rendering it unavailable to the plants' roots.
The clearest sign of iron deficiency (a condition known as chlorosis) is leaves whose blades are yellow, but whose veins are green. If however, the leaves are perfectly green, the indication is that iron is definitely not lacking. Applying iron fertilizer when it is not needed is not simply unnecessary, but liable to be damaging as well, since excessive micro-elements in the soil water can actually be toxic to plants.
Significant iron deficiency will seriously retard the plants, but the idea that fertilizer should be added to the soil in order to pre-empt the possibility of chlorosis is utterly false. Iron fertilizer should never be applied unless there are definite signs of chlorosis. This often occurs when building rubble has been buried in the soil, or when plants are close to a wall, and the lime in the cement residues raises the pH to levels where minerals crystallize and cannot dissolve. In general, the best way of securing soil conditions whereby the micro-elements are readily available to the plants, is to periodically add compost or humus to the soil, as this balances the pH towards neutral, (neither too acidic nor too alkaline) at least in the alkaline soils common to most dry climates.
Iron As a Plant Nutrient - In What Circumstances Should Iron Fertilizer Be Used?
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Jonathan Ya'akobi
Your Personal Gardening Coach