At the macro-scale biochar products range from powdery to brittle, depending upon the
physical microstructure of the material from which they are derived. Those produced from
woody feedstock display a predominantly xylemic structure that is coarse and strong. These
also display the highest carbon contents, in excess of 70% C and up to 90% C, and are low
in trace elements. Those produced from rye grass, maize and digested feedstocks i.e.
manures are powdery, lower in C (over 60%), and enriched in minerals and nutrients. Thus
the latter are not only less physically recalcitrant, they are also a more attractive microbial
substrate.