The use of Ca-modified, brown-coal-derived humates and fulvates for treatment of soil acidity

Abstract

Previous workers have demonstrated the amelioration of soil acidity with Ca-saturated, at least slightly soluble, organic materials. This study investigated the effectiveness of a new source of such materials, the humate- or fulvate-rich products of oxidation (hydrogen peroxide) of Loy Yang brown coal. The products were characterised by physical and spectroscopic measurements and by elemental analysis. Two products, one approximately half fulvate (fulvate-rich), the other predominantly humate (humate-rich) were mixed with CaCl₂ and then used to treat columns of an acid red podzol soil from Book Book, NSW. Leachate fractions were collected and analysed for pH, ionic strength, and concentration of Ca, Mg, K, Mn, Fe, and Al. After leaching (3 pore volumes), each soil column was sectioned and exchangeable Ca and Al contents were determined. The results were compared with those obtained from lime-treated and untreated columns.

The Ca-fulvate-rich product was the most effective in lowering the exchangeable Al content of the soil whilst increasing exchangeable Ca and soil pH. About 90% of the reduction in exchangeable Al was due to loss of Al in the leachate rather than immobilisation. The pH increased over the top 15 cm of the column from 3.8 ± 0.1 to 4.5–5.4. The Ca-humate-rich oxidised product was less effective in ameliorating soil acidity, as expected from its chemical properties, but it increased soil pH (to 4.0–4.7) and exchangeable Ca in the top 6 cm of the column and decreased exchangeable Al in the top 12 cm. More of the reduction in exchangeable Al than for the treatment with Ca-fulvate-rich material was due to conversion to an immobile form. This material was superior to lime, which only gave significant improvement near the soil surface. The leachates from treated columns contained the same amounts of Mg, Mn, and K as a control, but more Fe was leached from treated columns, particularly that treated with the Ca-fulvate-rich product.