Reactions of iron and carbonates in calcareous soils

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Abstract
Fe chlorosis is commonly associated with calcareous soils. Calcium carbonate has a dominating influence on any system in which it is present due to its properties of high solubility, high buffer capacity and basicity. At the pH of a calcareous soil, approximately 7.4 to 8.5, the equilibrium concentration of total dissolved Fe(III) is approximately 10‐10 M, which is considerably less than the 10‐8 M concentration which is required for optimum growth of plants in nutrient culture. Therefore, any plant in a calcareous soil is likely in an Fe stress situation. The plant may respond to the Fe stress by exudation of proton and chelate and/or development of an increased capacity for reduction of Fe(III) to Fe(II). These response factors will act to mobilize labile Fe from solid phase soil components, especially the poorly crystalline Fe oxides. Therefore, the crystallinities, particle sizes and reactive surface areas of the soil Fe oxides influence the availability of Fe to plants in calcareous soils. Other soil factors may either enhance the stress condition or influence the effectiveness of the plant stress response. One of the major factors is soil solution bicarbonate concentration, which is controlled predominantly by the equilibrium relations of soil carbonate phases with soil atmospheric carbon dioxide. High partial pressures of carbon dioxide are more likely under conditions of poor drainage, high soil‐water content, soil compaction, and/or high microbial respiration rates. Fe chlorosis is also known to be more prevalent under the above conditions. These reactions and their implications to the management of Fe chlorosis are discussed in this paper.

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