Estimating the Lime Equivalence of Biochar for Quality Assessment

The lime equivalence of biochar indicates its acid neutralizing capacity when applied as a soil amendment. The International Biochar Initiative recommends lime equivalence among other parameters for assessing the quality of biochar that is used in soil. To understand the factors controlling the lime equivalence of bichar, twenty biochar products commercially or custom manufactured from diverse feedstock materials and pyrolysis operations were measured for lime equivalence using back titration methods. The lime equivalence measurements were regressed against other biochar characteristics including pH, electrical conductivity (EC), water soluble base cations (WSBC), and ash content. The biochar products contained 3.6–60.8% of mineral ash and demonstrated pH, EC (1:10 solid/water extract), and WSBC in ranges of 6.0–11.5, 0.22–15.5 dS m-1, and 31.2–1619.1 meq kg-1, respectively. The lime equivalence of the biochar products ranged from -84.7–118.3 g CaCO3 kg-1. While the EC, WSBC, and ash content were highly correlated with each other (R2 >0.95), the pH value did not follow a clear trend with any of the parameters. The lime equivalence of biochar was not in relation to its pH value. For two biochar specimen with pH at 10.2 and 10.3, their lime equivalence values were -35.0 and -23.1 g CaCO3 kg-1, respectively. Evidently, the pH and lime equivalence of biochar are influenced by other factors in addition to the inherent ash minerals or WSBC. Biochar may contain significant active and residual acidities originated presumably from the organic acid condensates generated during pyrolysis of biomass. Further research is warranted to identify optimal pyrolytic conditions for minimizing the acidity of biochar products.

Keywords: biochar, lime equivalence, residual acidity, mineral ash, titration

Mingxin Guo
Delaware State University, Dover, DE 19901, United States