Subject: Pineapples and the iron content of Hawaiian soils

Is it true that the pineapple farmers must apply iron (Fe) to the soils because the native volcanic soils are deficient in iron? A friend of mine who visited an Agriculture research station in Hawaii some years back said that the Hawaiians actually dissolved old cars in acid to get a soluble form of iron for fertilizing pineapples. True or False?

    Your question is an interesting one. I have learned from Robert E. Paull of the UH Dept. of Tropical Plant & Soil Sciences that early in the twentieth century pineapple companies did dissolve parts of cars in sulfuric acid to obtain iron sulfate to use on pineapple plants. This iron was not applied to the soil, where it becomes chemcially, bound up. Instead, it was applied, and still is (but not from cars), to foliage in all commercial pineapple fields on a regular schedule, otherwise the plants turn yellow and grow poorly.
   Interestingly, Hawaiian soil generally has abundant, but not bioavailable iron (depending on soil age). The soils in Hawaii primarily derive from the decomposition of two rock types: basalt (which has roughly 10% by weight Fe) and coral (which is entirely calcium carbonate). The latter only occurs in very specific locations (mostly coastal) and are not typically utilized for agriculture. There are also soils that are a mixture of the two types, but typically the break-down product of basalt dominates.
    The process through which rock becomes soil is highly climate dependent, as the amount of water and organic matter present during this process determines the relative abundances of the soluble (e.g., Na, K, P), moderately soluble (Ca, Fe, Mg) and insoluble (Al, Si and Ti) components of the rock that make it into the soils. In cases of very extreme weathering in the presence of large amounts of water (i.e., in rain forests), a soil type composed largely of aluminum oxide is formed. This soil would require the addition of Fe and a large number of other nutrients for it to commercial agricultural products, however, the areas where these soils occur are also not prime agricultuiral growing areas, due to the near-constant presence of water.
    Pineapples are typically grown in large fairly flat tracts of well-drained inter-montain land that is composed of what is affectionately termed Hawaiian red dirt (the red color is due to the presence of the oxides of Fe). Although Fe is farily abundant in many Hawaiian soils, it is poorly available to pineapple plants because of the minerals it occurs in and because pineapple has a limited root system. (so other plants with more extensive root and ion uptake systems can extract sufficient iron for their needs from Hawaiiain soils). So this is why it was and still is added to pineapple plants today.

Dr. Ken Rubin, Professor
Department of Geology and Geophysics
University of Hawaii

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