Physical and Chemical Properties and Plant Growth in an Engineered Soil Manufactured from Bauxite Residue, Green Waste Compost and Increasing Amounts of Sand

Abstract

A new manufactured soil product (Turba) was produced using acidified bauxite residue into which 10% green waste compost had been incorporated. A laboratory/greenhouse experiment was carried out to determine if sand could be used as an ingredient or an amendment for Turba. Sand was added at rates of 0%, 5%, 10%, 25, 50% and 75% (w/w) in two different ways 1) by incorporating it into the Turba during its manufacture (IN) or 2) by mixing it with Turba aggregates after their manufacture (OUT). Incorporation of sand into Turba aggregates (IN) decreased the percentage of sample present as large aggregates (2 - 4 mm dia.) after crushing and sieving (<4 mm) and also reduced the stability of 2 - 4 mm dia. formed aggregates (to dry/wet sieving) and are therefore not recommended. In a 16-week greenhouse study, ryegrass shoot yields were greater in Turba than in sand [and decreased with increasing sand additions (OUT)] while root dry matter showed the opposite trend. The greater grass growth in Turba than sand was attributed to incipit water stress in plants grown in sand and this may have promoted greater allocation of assimilates to roots resulting in a greater root-to-top mass ratio. The much lower macroporosity in Turba coupled with the solid cemented nature of Turba aggregates resulted in production of thinner roots and therefore greater root length than in sand. Turba (manufactured from bauxite residue and compost added at 10% w/w) is a suitable medium for plant growth and there is no advantage in incorporating sand into, or with, the Turba aggregates.