Comparative Physical Properties of a Manufactured Soil Based on Bauxite Residue and a Degraded Natural Soil

Abstract

A new manufactured soil product (Turba) was produced by acidification of bauxite residue and the addition of 10% green waste compost. The physical properties of this material were compared with that of a natural soil (from under long-term cropping) after 6 months under either fallow or ryegrass. After 6 months under fallow or grass, aggregate sizes separated by sieving were less than the original sieved sizes for Turba and greater for soil. However, wet and dry sieving of 2 - 4 mm dia. aggregates showed Turba aggregates were more stable than those of the soil. Turba aggregates were strongly bound together due to a one-off pozzolanic cementation caused by the presence of Ca aluminates and silicates. The ratio of mean weight diameter for wet sieving to that for dry sieving was less than 0.4 for the soil and greater than 1.0 for Turba. Turba aggregates were extremely stable in water but brittle and could be decreased in size by mechanical action. The ratio was increased by the presence of plants for soil but was unaffected for Turba. Root growth and associated soil microbial activity had no measurable effect on aggregate size or stability in Turba because roots grew between rather than through the solid, compact, cemented Turba aggregates. Turba aggregates had high microporosity and most of the macroporosity occurred between aggregates. The larger the aggregates the larger the interaggregate pores and the greater is the macroporosity. Minimization of disturbance during use (permanent turf, forage or plantation crops and use of minimum tillage) will be important management considerations for the use of Turba.