Identifying biochar production variables to maximise exchangeable cations and increase nutrient availability in soils

Biochar is a product of the thermal treatment of sustainably sourced organics or waste biomass, which enhances nutrient availability and exchangeable cations after soil application. Biochar feedstock is one of the primary factors influencing the availability of exchangeable cations in biochar and soil-biochar mix. However, this influence is not satisfactorily understood due to contrasting data among literature for the cation exchange capacity (CEC) and the lack of standard analysis methods for CEC of biochar and soil-biochar mixtures. Therefore, this research was conducted to assess cation exchange in contrasting soil-biochar mixes, where CEC is measured as (1) the sum of base cations (Na+, K+, Mg 2+, and Ca2+) displaced by ammonium acetate (CEC-BC) and (2) NH4+ displaced by potassium chloride (CEC-NH4+). For the first time, we have evaluated CEC-BC and CEC-NH4+ in 21 different biochar samples using a modified method. We then used 12 representative biochars with 3 soil types (2% w/w biochar) and provided insights on relative changes in CEC-BC and individual base cations. Some previous studies have indicated a relationship between CEC and surface area and a degree of additivity in the resulting CEC of soil-biochar mixes. However, our study demonstrates an absence of these correlations and a lack of additivity in the representative biochar products. We found that biochar prepared from mixed garden wastes can increase K+ or Ca2+ in loamy or sandy soils and mitigate potential soil sodicity problems in loamy soils. However, a clayey soil may or may not need biochar for the specific purpose of increasing exchangeable cations. Our results revealed that biochar prepared from hardwood or mixed garden waste can retain NH4+ and deliver it to soil acting as a sink and source for soil nutrients. [Display omitted] •Presence of exchangeable cations in biochar is primarily influenced by feedstock type.•Exchangeable K+ was increased by more than 400% in sandy loam soil after application of 2% w/w biochar.•Hardwood or cellulosic biomass derived biochar increases the cation exchange capacity of clayey soils.•Biochar can help mitigate potential soil sodicity by reducing exchangeable Na+ in loamy soils.•Biochar can store significant NH4+, suggesting potential use as a slow-release fertilizer.