The most important measures of biochar quality include
adsorption, cation exchange capacity, mobile matter (tars,
resins, and other short-lived compounds) and type of
organic matter feedstock used. Over time, adsorption capacity
of biochar decreases, whereas its cation exchange
capacity increases. Mobile matter can block porosity and initial
adsorption but is highly susceptible to biological decay,
which can mitigate those effects. The physical structure
of the feedstock, mainly its pore size, which greatly
determines surface area, water retention, and biological
utilization of the biochar produced, is essentially locked
into form during “thermal modification.” While a greater
proportion of micro-pores may yield a higher surface
area, and thus greater nutrient retention capability, many
soil microorganisms are too large to utilize such small
spaces and benefit from some amount of larger pore
sizes. In terms of increasing plant
growth, biochar with various pore sizes may be best
suited to enhance the physical, chemical, and biological
characteristics of soils.