Boron contents and solubility in Australian fly ashes and its uptake by canola (Brassica napus L.) from the ash-amended soils

Phytotoxicity due to excessive boron (B) uptake by plants impedes routine agronomic utilisation of coal fly ash. We assessed 11 fly ashes (pH 3.14-10.77) having total B content ([B.sub.t]) of 12-136 mg/kg, of which 20-30% was hot water soluble ([B.sub.s]) in the acidic ashes (pH We conducted two pot trials in which canola was grown in soils amended with fly ash. In the first trial, an alkaline fly ash ([B.sub.t] 66 mg/kg) was incorporated at 5 rates of up to 625 Mg/ha into the top 50 mm of 2 acidic soils in 0.30-m-long intact cores, and sown with canola. Boron concentration in leaves at flowering reached the phytotoxic threshold, and both plant growth and seed yield were reduced, only at 625 Mg/ha. In the second trial, 4 fly ashes (pH 3.29-10.77, [B.sub.t] 12127 mg/kg) were incorporated at 4 rates of up to 108 Mg/ha into the top 0.10 m of 2 acidic soils in 1.0-m-long intact cores and then sown with canola. Ashes with highest [B.sub.t], when applied at 108 Mg/ha, increased B concentration in the topsoil only. Of the 2 ashes with the highest [B.sub.t], only that which produced low soil pH and applied at 108 Mg/ha increased B concentration in the shoot, but was still below phytotoxic threshold. The results suggest that B derived from these ashes may not cause phytotoxicity and excessive soil B accumulation if the ashes are applied at modest rates ( Additional keywords: trace elements, boron toxicity, soil boron, soil pH.
Introduction Coal fly ash is a useful soil amendment for supplying plant and animal nutrients and improving soil physical and chemical properties. However, certain trace elements in fly ash can [...]