Biochar as a potential soil additive for improving soil physical properties—a review

Biochar, a solid product produced from biomass pyrolysis under low oxygen conditions, has gained wide acknowledgment in its usage as a means for carbon sequestration as well as improving the soil chemical and physical properties of the soil. Although the effects of biochar application on chemical characteristics and fertility of soils have been intensively investigated, there is little information on its role in improving soil physical characteristics. Therefore, this review aimed to (i) summarize the impact of biochar application on soil physical properties, (ii) discuss the factors and mechanisms influencing biochar performance on soil physical properties, and (iii) identify future research priorities. This review concluded that the improved impact of biochar application on soil physical characteristics is dependent upon feedstock and pyrolyetic conditions of biochars, application rate of biochar, biochar particle size, and soil type and texture. Pyrolysis temperature is the main factor controlling biochar properties such as porosity and surface area, which reflect their effects on soil physical characteristics. For the same feedstock, the temperature will control the properties of resulting biochars. But, the biochar properties greatly depend on the properties of feedstock. For example, manure-derived biochars contain a large amount of ash, but biochars from cellulose-lignin biomass mainly consist of the carbon fraction. Despite the profound effect of biochar in improving the physical properties of soil, the economic impact of its implementation in large-scale farming has not been established. Therefore, there is need for its economic evaluation.