Does biochar application alleviate soil compaction? Review and data synthesis

Biochar is considered to enhance numerous soil ecosystem services, but whether it alleviates soil compaction is not well known. This paper: 1) synthesizes the biochar impacts on soil compaction parameters including bulk density, penetration resistance, aggregate tensile strength, Proctor maximum bulk density (soil compactability), and the water content at which the Proctor maximum bulk density occurs (critical water content), 2) discusses potential factors affecting biochar performance, and 3) compares the impacts of biochar on soil compaction with those of other amendments. Literature indicates that biochar generally reduces bulk density, tensile strength, and Proctor maximum bulk density; and increases the critical water content but may or may not reduce penetration resistance. Biochar reduces bulk density from −1% to −20% (-8.8% average), tensile strength from −8 to −72% (-31% average), and Proctor maximum bulk density from −1 to −19% (-9.3% average), whereas it increases the critical water content from 1 to 64% (21% average). The increase in critical water content suggests soils with biochar can be trafficked at higher water content without causing compaction than soils without biochar. An increase in biochar application rate reduces soil compaction and explains 11% to 63% of the variability in compaction parameters, but biochar benefits can be short-lived (<2 yr). It appears that, in general, at least 10 Mg ha⁻¹ of biochar are needed to significantly reduce compaction. Larger application rates (>20 Mg ha⁻¹) can be needed for reducing penetration resistance and Proctor maximum bulk density than for other compaction parameters. Biochar has similar benefits to manure for reducing soil bulk density and can complement other organic amendments for reducing soil compaction. How biochar feedstock, pyrolysis temperature; and companion amendments; and soil texture influence biochar impacts on compaction is still unclear. Overall, biochar has the potential to alleviate soil compaction, but more long-term field data, particularly on penetration resistance, Proctor maximum bulk density, and critical water content for multiple biochar application rates, feedstocks, pyrolysis temperature; and amendments are needed to better elucidate biochar effects, biochar longevity, and optimum levels of biochar application.