Amelioration of acidic soil with an ettringitic by-product of waste water treatment

Abstract

Ettringite (Ca₁₂AI₁₄(OH)₂₄(SO)₆ 52H₂O) is the main by-product of a saline water beneficiaron process currently being evaluated by coal-based industries. The study aimed to quantify and explain the benefits of using this alkaline waste to ameliorate acidic soils by comparing its effect on plant growth with those of calcite and gypsum. A four-week growth experiment in a phytotron was conducted with maize (Zea mays L.) in 1.2 kg pots of acidic, sandy soil (pH 3.5 in 1/WKCI) using treatment levels of CaCO₃, CaSO₄. 2H₂O and an ettringite-dominated waste (EW) containing up to 2 g Ca per kg soil. Yield of dried maize shoots was unaffected by gypsum but responded significantly to calcite and EW, confirming the potential value of EW as a liming material. Soil pH and 1M KCI-extractable acidity correlated strongly with yield; acid saturation did not. Soil solution composition in the EW-treated soif was consistent with that which would promote the formation of a co-precipitate of phosphate with Al or with both Al and Ca. Foliar analysis showed variable nutritional responses to the treatments and indicated that applied Ca level was a better means of predicting shoot Ca concentration than were indices of soil Ca availability (extractable in water or KCl). An excess of P was indicated by both soil and plant analyses, and may possibly have contributed to plant growth responses to soil amendment through co-precipitation of Ca phosphate with other ionic solutes and the creation of plant nutrient imbalance.