Bauxite residue (red mud) improves pasture growth on sandy soils in Western Australia

Abstract

Red mud is a finely crushed, iron-rich, alkaline residue, obtained by digesting bauxite with caustic soda to remove the alumina. The remnant alkalinity of red mud is equivalent to 11% pure calcium carbonate.

Phosphorus leaching from infertile sandy soils has resulted in eutrophication of estuaries and has caused algal blooms. Red mud has been shown to reduce leaching of phosphorus from sandy soil. This research was undertaken to determine the effect of red mud on pasture growth and uptake of heavy metals.

Red mud, either untreated or treated with gypsum, was applied at rates of 0, 10, 20, 40, and 80 t/ha to a subterranean clover and ryegrass hay paddock. There were 3 replicates of each treatment and a completely randomised design was used. The experimental design was 5 rates of red mud x 2 untreated and treated with gypsum x 3 replicates, resulting in 30 plots. Plant growth, and nutrient and heavy metal composition of the plant tops, were measured.

An application of 40 t/ha of red mud increased hay (mainly subterranean clover and ryegrass) production by 24% and increased soil pH in the top 10 cm by 1.0 unit from 3.5 (1 : 5 soil : 0.1 M CaCl₂). The increase in production was probably because of the liming effect of the remnant alkali in the red mud, which may have potential as a replacement for crushed limestone. Sodium carbonate, the predominant alkali in red mud, is more soluble than calcium carbonate from crushed limestone and has the potential to change the pH of the soil more rapidly. The soil was top-dressed with red mud, without disturbing the existing pasture, resulting in changes to the pasture production and nutrient composition consistent with a change in soil pH throughout the rooting depth. If crushed limestone is not mixed into the soil it may take many years to increase the pH of the soil; however, this mixing results in extra cost from re-seeding and an initial depression in yield. Although much more red mud is needed than lime, the cost is comparable when the haulage distance is less than about 30 km.

In previous trials, at red mud application rates > 500 t/ha, gypsum was mixed into the red mud to reduce salinity and pH. At these rates, the red mud had overwhelmed the buffering capacity of the soil. The gypsum reduced the pH by changing the sodium carbonate in the red mud to calcium carbonate, thus changing the pH from > 10 to about 8.5. However, amendment of the red mud with gypsum when applied at rates < 80 t/ha proved unnecessary in this experiment, probably because the proportion of soil was sufficient to alter the pH of the red mud. When red mud is applied to acidic infertile sands, manganese application as manganese sulfate may be necessary, because the rise in pH may rapidly induce manganese deficiency in plants. Care should be taken to monitor other nutrients which have their availability for plants affected by pH (e.g. copper, zinc, and molybdenum). Red mud did not elevate the concentrations of heavy metals in the soil, hay, or fresh plant tissue.