Water-extractable alkalinity to estimate the acid-neutralising capacity of organic amendments

Abstract

The effectiveness of amendments to increase the pH of an acidic soil can be expressed as acid-neutralizing capacity (ANC). The alkalinity measurement method, based on prolonged hydrolysis of organic amendments with acid, is widely used but may overestimate the ANC of the amendments. We developed methods for determining the filtered water-extractable alkalinity (WEA) and unfiltered WEA of organic amendments to provide more accurate estimation of their ANC. Filtered WEA indicates the alkalinity arising from water-soluble compounds, while unfiltered WEA relates to alkalinity originating from both surface adsorption of proton by particulate organic materials and water-soluble compounds. The methods were tested using organic materials differing in decomposability; readily decomposable wheat straw, faba bean straw, and more resistant blended poultry litter, biochar, and compost. Filtered WEA of the organic amendments obtained at equilibrium extraction time (12 h) ranged from 6.5 cmol H⁺ kg⁻¹ (wheat straw) to 14.2 cmol H⁺ kg⁻¹ (biochar), whereas the unfiltered WEA measured after 2 h of extraction with water and short acid treatment during titration ranged from 10.3 cmol H⁺ kg⁻¹ (wheat straw) to 219.3 cmol H⁺ kg⁻¹ (compost). Unfiltered WEA values were slightly lower than acid-extractable alkalinity in rapidly biodegradable materials, but significantly lower in resistant organic materials. This could be because the former materials breakdown readily in water to release organic alkalinity, whereas resistant materials decompose and release organic alkalinity slowly in water, compared to that induced by prolonged acid hydrolysis. The organic materials treated with HCl produced stronger FTIR absorption peaks at various bands than those treated with water. Hydrolysis in water did not cause significant changes in spectral peaks compared to the original organic materials. Rates of organic amendments or organic amendment-lime combinations calculated for acidic soil (pHw 4.8) based on unfiltered WEA results in a pHw closer to 6.5. We conclude that the unfiltered WEA method is suitable for determining the available alkalinity of organic materials that can be used for estimating amendment rates for amelioration of acidic soils.