SummaryAerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat‐clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite‐derived ochre results from the action ofThiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe2+and pyrite. In soils less acid thanc.pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe2+and SO24−. Under these conditions iron enters the drains as Fe2+and is there oxidized byT. ferrooicidansand deposited as hydrated ferric oxide. Once the soil becomes acid enough forT. ferrooxidansto multiply, the rate at which pyrite oxidizes increases several‐fold, and atc.pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of