AN EXAMINATION OF THE BIOLOGICAL REDUCTION METHOD FOR ESTIMATING ACTIVE IRON IN SOILS

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Abstract
Summary: Allison and Scarseth's biological reduction method was compared with three chemical methods for the removal of iron and aluminium from six Scottish surface soils. When applied to soils which had been pre‐treated with hydrogen peroxide and inoculated with Fe‐reducing bacteria the amount of Fe dissolved by this method was intermediate between Tamm's acid‐ammonium oxalate, the mildest chemical method, and Mackenzie's hydrosulphite method. When applied to untreated soils the amounts of Fe dissolved were sometimes as little as 15 per cent of that removed from peroxide‐treated soils. With the latter soils the amount of soluble Fe increased as the amount of sucrose added to the soil and as the duration or temperature of incubation increased. Al also became more soluble as sucrose increased and the pH of the fermentation decreased. The peroxide treatment rendered A1 more soluble but this could be largely counteracted by increasing the pH before fermentation. The method could be made about five times more specific for Fe than the hydrosulphite method, the most specific of the chemical methods, eight times more than Truog's nascent hydrogen sulphide method, and fourteen times more than the acid‐ammonium oxalate method.The amounts of Fe dissolved by the biological method gave little indication of phosphate sorption capacity and seemed inferior in this respect to acid‐oxalate soluble Fe. A1 dissolved by all methods, especially the chemical ones, was highly related to phosphate sorption.
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