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The use of the fine residue from bauxite refining as a soil amendment

Ward, Samuel Campbell (1986) The use of the fine residue from bauxite refining as a soil amendment. PhD thesis, Murdoch University.

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Large quantities of the alkaline residue from bauxite refining are produced each year in Western Australia. If alternative uses could be found for the residue this would decrease the amount of land required for storing the residue and reduce the problems of managing these storages. This study investigated the use of the fine fraction of the residue (red mud) as a soil amendment for sandy soils. Red mud has a high capacity to retain water and nutrients, particularly phosphorus. Amending sandy soils of the Swan Coastal Plain with red mud could increase their productivity and reduce the leaching of phosphorus fertiliser. The leaching of phosphorus fertiliser from these sandy soils has been directly implicated as the cause of eutrophication of coastal water bodies (Dept. Cons. Env. W.A., 1984).

The growth and persistence of annual legume pastures on sandy soils amended with a range of levels of red mud or red mud ameliorated with gypsum (red mud/gypsum), up to 2000 t/ha, was investigated at four sites on the Swan Coastal Plain. The fertiliser requirements of the legume pastures growing on amended soils were determined. The physical and chemical characteristics of the amended soils and changes in these characteristics with time were investigated. The concentrations of a range of elements in the plants growing on amended soils were determined to identify possible nutrient deficiencies or potentially hazardous levels of elements.

On well-drained, coarse sands amendment with red mud, neutralised with 5% gypsum, increased total pasture growth by up to 100%. Increases in the legume component of the pasture were even greater. Amendment with 500t/ha or more of red mud/gypsum, in the top 20-30 cm of soil, is recommended for these sands. These levels of red mud/gypsum amendment significantly increased the water-holding capacity of the sands and reduced their water repellence. Consequently yields were highest and legume pastures should persist.

On low-lying, coarse, acid sands amendment with red mud/gypsum did not always increase pasture growth. At 500 t/ha, or less, pasture growth was sometimes increased but never decreased by amendment. On these sands 200 t/ha of red mud/gypsum, or a minimum consistent with controlling the loss of phosphorus from these soils, is recommended. When 200 t/ha of red mud/gypsum was applied, pasture yields were always high, legume seedlings survived well during extended dry periods in autumn, soil pH was raised to about 6.5 and there should be sufficient red mud/gypsum to reduce phosphorus leaching.

Pastures growing on soils amended with red mud/gypsum required phosphorus and potassium fertiliser, but only at normal agricultural application rates. There was a response to a foliar application of manganese fertiliser, in some cases.

With one exception, there was no evidence of any elements accumulating to levels toxic for animals or plants in pastures growing on soils amended with red mud/gypsum. The exception was molybdenum, when high rates of molybdenum fertilisers were applied. Molybdenum containing fertilisers should not be applied to soils amended with red mud.

After 3-4 years in the field, soils amended with red mud/gypsum were sufficiently leached to be classed as non-saline, or at worst mildly-saline, and non-sodic. At levels of red mud/gypsum application of 200-500 t/ha or more, the pH values of the amended soils were around 8.3 indicating that they were being buffered by calcium carbonate.

Although red mud/gypsum amendment was generally beneficial for pasture growth, the costs of amendment are too high to be economic. Large-scale amendment of soils is only likely if the taxpayer or alumina companies bear the majority of the cost. Red mud/gypsum amendment deserves more consideration as a strategy to reduce the inflow of phosphorus into the Peel Inlet and Harvey Estuary, Western Australian to overcome the eutrophication problems.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Barrow, Jim and Ho, Goen
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