PHYSICAL PROPERTIES OF MANUFACTURED SOIL (TURBA) PRODUCED FROM BAUXITE RESIDUE

Bauxite residue was treated by a method developed by Emirates Global Aluminium (EGA) involving acidification and removal of excess salts to produce Optimized Bauxite Residue (OBxR). Composted green waste was added to the wet OBxR at a rate of 10%. The products were dried, crushed and sieved and the physical properties of the material characterized. Compared to dune sand, Turba had a greater total porosity, mesoporosity and microporosity and a lower macroporosity. Plant available water was 130 -230 kg m-3 which was about 4 times greater than that of dune sand. In comparison with a soil taken from under intensive cropping, Turba had a much greater microporosity and lower macroporosity and aggregates were extremely stable as estimated by wet and dry sieving. Ryegrass was grown in sieved Turba aggregates ( < 2 and 2-4 mm dia), soil aggregates ( < 2 and 2-4mm dia), and sand for 6 months. It was noted that roots grew predominantly between the solidified Turba aggregates rather than through them (as occurred for the soil) and there was no evidence to suggest root growth promoted aggregation of smaller Turba aggregates (<2 mm dia.) into larger (2-4 mm dia.) ones. In fact, aggregate diameter tended to decrease over time in Turba. Such results reflect the fact that in Turba aggregation is a one-off solidification reaction that occurs upon drying for the first time (caused by the cementing actions of pozzolanic Ca silicates and aluminates). By contrast, in soils aggregation is a dynamic on-going process which is promoted by root growth. Management of Turba as a growing medium will need to take into account the different physical nature of Turba compared with field soil and measures that maintain aggregate integrity and physical condition will need to be instituted such as use of permanent vegetation (e.g. grass and plantation crops) and use of minimum and zero tillage when growing annual crops.