To develop an assay for bioavailability of micronutrients in soils.

• The commonest deficiency detected was that of zinc, followed by copper, then by molybdenum, but deficiencies of iron and manganese were rarely found. Usually surface soils with a well-developed root zone were less deficient than subsoil.
• There was a fair correlation between the availability of zinc, copper, and molybdenum to A. niger (M.) and availability to plants, but none for iron and manganese • The fungus was able to obtain adequate amounts of micronutrients from a variety of insoluble salts or oxides of iron, zinc, copper, manganese, and molybdenum, some of which were known to be unavailable to plants .
• A number of soils inhibited the growth of the mould in complete medium. This effect was due to active fixation of added micronutrients, especially in soils rich in calcium carbonate and in "red loams".
• No evidence was obtained that microbial fixation of trace elements occurred in deficient soils; but most major and minor elements were fixed by microorganisms in very deficient, acid-washed, quartz crystals, with the exception of calcium, which was not significantly affected, and of iron and zinc, which were released by microbial activity from the minute amounts of contaminants present.