Comparison of Movement of Alkalinity From Surface Applied Biochar, Wood ash and Dolomitic Limestone in Acid Sandy Loam Soil

Amelioration of subsoil acidity in established perennial crops is difficult as agricultural limestone cannot be incorporated. Biochar and wood ash contain other sources of alkalinity besides calcite or dolomite, that are potentially more soluble. Therefore, the objective of this controlled study was to compare the nature and mobility of alkalinity from surface applied biochar, wood ash and dolomitic limestones. Black wattle biochar, pine wood ash and agricultural and granulated dolomitic limestones were characterized using X-ray diffraction, scanning electron microscopy, calcium carbonate equivalence (CCE) and elemental analysis. In a column study, the liming materials were surface-applied to acid sandy loam soil (pHKCl 4.3) according to the incubation lime requirement to achieve a target soil pHKCl of 5.5, then 1000 mm simulated irrigation applied. Soil pHKCl and basic cations were then determined in 50 mm depth increments. Dolomitic limestones achieved target soil pH at 0–50 mm only. Wood ash alkalinity was more mobile achieving target pH at 0–150 mm, attributed to mixed basic oxides, carbonates and silicates. Biochar alkalinity was most mobile, achieving target pH ± 0.2 pH units throughout the 400 mm soil column, attributed to micro-fine K and Ca carbonates. The superior mobility of alkalinity from biochar and wood ash demonstrate their potential use in amending subsoil acidity. However, considering the low CCE of the biochar (6%) and wood ash (24%), substantially larger, unrealistic quantities of these materials will be required compared to limestone. Furthermore, the higher solubility of K compounds in the biochar led to serious basic cation imbalances.