Alkaline mineral residues from pulp mills as a sustainable and economical alternative to lime fertilizers

Abstract

Lime is typically comprised of calcium carbonate (CaCO₃) or calcium magnesium carbonate (CaMg(CO₃)₂) and is needed to maintain a proper pH in agriculture and forestry soils, but it represents a major cost to growers and results in significant greenhouse gas emissions due to the mining, crushing, and transport required. There is a need to identify existing sources of alkaline mineral wastes and assess their potential as sustainable and cost-effective alternatives to lime. Dregs and grits (DGs) are highly alkaline mineral residues from biomass pulp mills that form during the recovery of pulping chemicals and are mostly comprised of CaCO₃. For the first time, we assess the efficacy of industrially sourced DGs on 22 acidic soils across the Southeastern United States that are currently used in either agriculture or forestry operations. The application of DGs was found on average to result in the same soil pH response as calcite over multiple soil incubations (0 day to 120 days). An analysis of the carbon content of the soils after the incubation revealed that soils incubated with DGs experienced a statistically significant increase in soil carbon relative to control soils and those incubated with CaCO₃. The generation and application of DGs emits 0.35 tonnes of CO₂ for every tonne of DGs, which compares favorably to the 0.86 tonnes of CO₂ for every tonne of agricultural lime generated and $85 per tonne lime reported in the literature. DGs prove to be a sustainable and economical substitute for agricultural lime in the acidic soils of the Southeastern United States.