Bioassay of available trace metals from Australian soils

Abstract

A study was made of various factors influencing the microbiological assay of available iron, zinc, copper, manganese, and molybdenum. Aspergillus niger (M.) was chosen as the best test organism from a group of 9 fungi, 1 alga, 1 yeast, and 6 bacteria. The composition of the basal medium could be altered markedly without affecting the availability figures. A new method of purifying the medium with A1203, gave good deficiency symptoms for Fe, Zn, Cu, and Mn, but Nicholas's method was superior for removing Mo. It was found necessary to sterilize the soil, preferably by autoclaving. The availability of 15 micronutrients was determined by bioassay from 278 surface and 164 subsoil samples from eastern Australia. Seven soil groups, viz. 3 pedalfers, "red loam", acid humic, and podzol, and 4 pedocals, rendzina and terra rossa, chernozem, red-brown earth, and desert soil, showed more deficiencies than all other soil groups bioassayed. Even within these seven groups non-deficient soils also occurred. The commonest deficiency detected was that of Zn, followed by Cu, then by Mo, hut deficiencies of Fe and Mn were rarely found. Usually surface soils with a well-developed root zone were less deficient than subsoils. Fewer deficient samples were obtained from meadow soil, alkaline humic, volcanic tuff, and brown earth than from all other soil groups. There was a fair correlation between the availability of Zn, Cu, and Mo to A. niger (M.) and availability to plants, but none for Fe and Mn. The fungus was able to obtain adequate amounts of micronutrients from a variety of insoluble salts or oxides of Fe, Zn, Cu, Mn, and Mo, some of which were known to be unavailable to plants. A number of soils, e.g. red loam, laterite, and desert soil, 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 Ca, which was not significantly affected, and of Fe and Zn, which were released by microbial activity from the minute amounts of contaminants present.