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 (LBCeq) 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 pHw 4.84 and pHCa 4.21. The LBCeq 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 pHw 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 pHw 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 pHw 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 pHw 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.