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

Abstract. 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), based on the titratable alkalinity of OA 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 % field capacity (FC). The soil used to develop and validate the method was a sandy loam with pH_w 4.84 and pH_Ca 4.21. The LBC_eq of the soil was 1657 mg CaCO₃ kg^-1 pH^-1. 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 soil pH_w to 6.48 and 6.42, respectively, whereas less biodegradable or resistant OAs (ROAs) (i.e. blended poultry litter, biochar, and compost) had lower pH values. At 150 % FC, the two straws increased soil pH_w to only 5.93 and 5.75, respectively, possibly due to slower decomposition in submerged conditions, resulting in limited alkalinity production, whereas amendment with ROAs produced pH_w values close to 6.5. With increasing lime equivalent value (LEV) of the OA, from 5.8 g CaCO₃ kg^-1 (wheat straw) to 179 g CaCO₃ kg^-1 (compost), the lime requirement to reach pH_w 6.5 in lime-OA combinations decreased from 2.72 to 0.09 g CaCO₃ kg^-1. The method was proved to be effective in determining appropriate rates of OAs (with or without additional lime) for management of acidic sandy loam soils in this study, but it needs to be validated for a particular soil and amendment.