Combining lime and organic amendments based on titratable alkalinity for efficient amelioration of acidic soils

Ameliorating soil acidity using a combination of lime and organic amendments (OAs) can be an alternative to lime alone, but determining the appropriate OA rates can be difficult. We developed a new method for calculating the combined application rate of lime and OAs (wheat straw, faba bean straw, blended poultry litter, biochar, and compost) that is based on the titratable alkalinity of OAs and the equilibrium lime buffer capacity (LBC eq ) of acidic soils. The effect of calculated soil amendment rates on soil pH was validated at soil water contents of 60 %, 100 %, and 150 % of field capacity (FC). The soil used to develop and validate the method was a sandy loam with a soil pH in deionised water (pH W ) of 4.84 and a soil pH in 0.01 M CaCl 2 solution (pH Ca ) of 4.21. The LBC eq of the soil was 1657 mg CaCO 3 kg - 1 pH - 1 (where "CaCO 3 kg - 1 pH - 1 " denotes the amount of lime required to raise the pH of 1 kg of soil by one unit). The titratable alkalinity of the OAs ranged from 11.7 cmol H eq + kg - 1 for wheat straw to 357 cmol H eq + kg - 1 for compost. At 60 % FC, faba bean and wheat straw amendment increased the soil pH W to 6.48 and 6.42, respectively, but lower pH values were reached in soil amended with less biodegradable or resistant OAs (ROAs) (i.e. blended poultry litter, biochar, and compost). At 150 % FC, the two straws increased the soil pH W to only 5.93 and 5.75, respectively, possibly due to slower decomposition under submerged conditions, resulting in limited alkalinity production, whereas amendment with ROAs produced pH W values close to 6.5. With an increasing lime-equivalent value (LEV) of the OA, from 5.8 g CaCO 3 kg - 1 (wheat straw) to 179 g CaCO 3 kg - 1 (compost), the lime requirement to reach pH W 6.5 in lime-OA combinations decreased from 2.72 to 0.09 g CaCO 3 kg - 1 . The developed method was shown to be effective in determining the appropriate rates of OAs (with or without additional lime) for the management of acidic sandy loam soils in this study and highlights the importance of the soil water content with respect to its acid-neutralising effect.