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

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Australian Journal of Soil Research, 2010, 48 (5), pp. 480 - 487
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Phytotoxicity due to excessive boron (B) uptake by plants impedes routine agronomic utilisation of coal fly ash. We assessed 11 fly ashes (pH 3.1410.77) having total B content (Bt) of 12136mg/kg, of which 2030% was hot water soluble (Bs) in the acidic ashes (pH 5) and 510% in the alkaline ashes, for their potential to supply B to plants and their risk associated with phytotoxicity. We found the Bs/Btto be negatively correlated (R2≤0.63, N≤11) with ash 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 (Bt66mg/kg) was incorporated at 5 rates of up to 625Mg/ha into the top 50mm 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 625Mg/ha. In the second trial, 4 fly ashes (pH 3.2910.77, Bt12127mg/kg) were incorporated at 4 rates of up to 108Mg/ha into the top 0.10m of 2 acidic soils in 1.0-m-long intact cores and then sown with canola. Ashes with highest Bt, when applied at 108Mg/ha, increased B concentration in the topsoil only. Of the 2 ashes with the highest Bt, only that which produced low soil pH and applied at 108Mg/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 (36Mg/ha) to the topsoil layers. © 2010 CSIRO.
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