Translocation of Iron In Acid Sulfate Soils: I. Soil Morphology, and the Chemistry and Mineralogy of Iron in a Chronosequence of Acid Sulfate Soils

In periodically flooded acid sulfate soils from Thailand various iron compounds are confined to distinct soil horizons. The unmottled substratum, which starts at several decimeters below the surface in the (youngest) mangrove and stage and at 2 m in well‐developed soils, invariably contains between 2 and 5% pyrite. Drainage of the mangrove mud is followed by pyrite oxidation which results in acidification of the soil and accumulation of iron as ferric oxide or jarosite at some distance above the pyritic substratum. Very little iron is precipitated just above the unmottled C horizon. As the soils become older and better drained, the upper boundary of the pyritic substratum and the B horizon with ferric oxide and jarosite, occur at progressively greater depths. Hydrolysis of pale yellow jarosite to ferric oxide at relatively shallow depth accounts for a separation of the B horizon in a brown mottled upper part and a predominantly yellow mottled lower part. The X‐ray amorphous ferric oxide is transformed to goethite, or, in the better drained soils, to hematite. In the older soils especially, periodic reduction of immobile ferric iron to mobile ferrous iron in the surface soil and at the top of the pyritic substratum, combined with perennially oxidized conditions in the B horizon, may contribute to the strong iron accumulation in the B horizon. Differences, in organic matter content and stability of ferric oxide, and the presence or absence of pyrite, probably account for the dissimilar behavior of the various horizons upon flooding. Besides conventional soil analyses, results of water analyses, E H ‐pH determinations, and various laboratory experiments involving field samples are helpful in explaining details of the soil‐forming process involved.