Synergistic Effects of Lime–Organic Amendment Interactions in Acidic Soils

ABSTRACT Calcitic lime (CaCO 3 ) is commonly used to ameliorate soil acidity, but its effectiveness is limited by slow solubility. This study aims to evaluate interactive effects of lime (90% calcite, 6% quartz) and organic amendments on lime dissolution and soil acidity neutralisation. The experiment utilised various organic amendments with the following differing decomposability: (i) readily decomposable faba bean straw, and (ii) more resistant blended poultry litter, biochar, and compost, with average organic carbon mineralisation indices (OC MI ) of 29%, 6%, 4%, and 6%, respectively. The lime–faba bean straw combination produced the highest partial pressure of CO 2 ( p CO 2 ) (1398 μatm) compared to lime alone (375 μatm). The net average increase in dissolved inorganic carbon (DIC) due to the synergistic interaction between lime and organic amendments ranged from +0.3 mg L −1 for the lime–blended poultry litter treatment to +1.9 mg L −1 for the lime–compost combination, representing a 2‐ to 5‐fold increase. The dissolved organic carbon (DOC) content in lime–organic amendment mixes also increased by 29%–36% compared to organic amendments alone. The p CO 2 /DIC ratio decreased when lime was combined with organic materials compared to when it was applied alone, indicating more efficient conversion of respired CO 2 to H 2 CO 3 * (* indicates this comprises both dissolved CO 2 and carbonic acid, H 2 CO 3 ). A lower DIC/carbonate alkalinity (DIC/CarbAlk) ratio and a positive calcite saturation index (SI C ) in the combined treatments further confirmed the increased generation of HCO 3 − and CO 3 2− , reducing acidity. The co‐application of lime and organic amendments also mobilised Ca 2+ while reducing potential Al 3+ bioavailability and phytotoxicity. These beneficial synergies highlight the potential for improved acid soil remediation strategies using combined lime–organic matter amendments.