Residues from bauxite‐mining (red mud) increase phosphorus retention of a joel sand without reducing yield of carrots

Abstract

Carrots were grown on a Joel sand amended with several levels of applied gypsum‐treated bauxite residue (RMG) up to 240 t‐ha^‐1, to test whether the residue reduces phosphorus (P) leaching when applied to the soil. Phosphorus sorption, measured using the Modified Phosphorus Retention Index (PRIM), was initially 30 at 2401 RMG ha^‐1 due to a combination of iron and aluminum oxides, calcium carbonate (CaCO₃), and soluble calcium (Ca). Four months after harvest, PRIM had decreased to 10 at 240 t RMG ha^‐1 (PRIM of 4 on unamended soil) due to the leaching of soluble Ca. Retention of fertilizer (P) (0–15 cm) at 160 kg P ha^‐1 increased from 34% on unamended soil to approximately 100% at 60 and 240 t RMG ha^‐1 one month after fertilizer application. Bicarbonate‐extractable P at harvest reached 60 to 65 mg‐kg^‐1 at 120 and 240 t RMG ha^‐1 when 346 kg P ha^‐1 was applied, whereas on unamended soil, levels did not exceed 30 mg‐kg^‐1, regardless of the level of applied fertilizer. Plant uptake of P was reduced due to the precipitation of calcium phosphate compounds, although final yield was unaffected possibly because of slow re‐release of P from precipitated calcium phosphate compounds. Red mud was difficult to wash off carrots grown on soil amended with 2401 RMG ha^‐1. The use of RMG may have a place in the management of horticultural crops in areas at risk from P pollution. However, more work is needed to investigate ‘aged’ RMG‐amended sites since the P retention in this experiment was affected by soluble Ca and also by post‐planting P applications.