Increasing iron availability to crops: Fertilizers, organo-fertilizers, and biological approaches

It has been known for 160 years that iron is essential for plant growth. Its deficiency occurs especially, but not only, in calcareous soils and it limits crop production in large parts (> 30%) of the Earth's arable land. The mineralogy and geochemistry of Fe in soils, as well as Fe functions within plants, soil factors that limit its availability, and plant mechanisms for Fe acquisition from soils are all fairly well documented. Yet, the alleviation of Fechlorosis and especially lime-induced chlorosis remains a major agronomic problem. Three main agronomic approaches to the alleviation of Fe deficiency will be discussed in this paper: 1) Increasing the availability of indigenous soil-Fe; 2) supplying the plants with external sources of available Fe; and 3) increasing plant efficiency in Fe uptake and translocation. The agronomic practices described include foliar Fe application; soil fertilization using inorganic Fe fertilizers, industrial by-products, synthetic iron chelates, and organoiron complexes; soil management and rhizosphere manipulation to increase the availability of indigenous soil Fe; and traditional and modern genetic approaches for increasing Fe-efficiency and acquisition by plants. New options for the alleviaton of lime-induced Fe-deficiency are described in detail. Emphasis is placed on the description of the mechanisms relevant to the effects of various treatments and approaches, with a view to generating further discussion and research that could lead to improved solutions for the problem of sources and alleviation of Fe-deficiency in crops.