Acidic iron oxide waste as a conditioner for calcareous soils

A fine-grained iron oxide (hematite, (α-Fe2O3), containing 1.2% hydrochloric acid by mass, is the main by-product of an acid recovery plant at Saldanha Bay. The alkalinity of calcareous soils in this semi-arid region causes plant deficiencies in many trace elements, including iron. The use of the acidic waste to decrease the pH of two sandy soils (one non-calcareous and the other containing 1.9% CaCO3) was evaluated in a pot trial. No significant differences were observed in the yield of young wheat in either soil treated with 0, 5 or 25 g iron oxide per kg soil. An application rate of 50 g/kg, however, suppressed wheat growth in both soils by about 40%. This was interpreted as being due to soil salinity in both soils (electrical conductivity, ECe, increased to about 10 dS m−1) and to acidification of the non-calcareous soil to pHKCI 4.04 (the calcareous soil remained buffered at pH 7).