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. As far as I know, the answer
is "false": Hawaiians do not dissolve old cars in acid to get a
soluble form of iron for fertilizing pineapples. It may be possible
that the research your friend spoke of did take place and was
directed toward enriching some particularly iron-deficient
soil type in Hawaii for agriculture uses, but in general our soils
are not deficient in Fe.
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 also exists 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 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 produce something like pineapples. However, the areas where these soils occur are also not prime pineapple (or other fruit) 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). So, although I can't rule out the possibility that someone once added Fe to their soil by the process you propose, it is generally not necessary as this element is actually farily abundant in many Hawaiian soils.
Dr. Ken Rubin, Asistant Professor
Department of Geology and Geophysics
University of Hawaii