The use of charcoal as a soil amendment and for CO2 sequestration raises many questions about the characteristics of those “biochars” and their impacts on soils and organisms. This paper reviews and revises the analyses of the principal characteristics used to distinguish biochars, and presents a small survey of measured properties. Explicit terminology is proposed about “resident and mobile carbon and other matter” in biochars intended for addition to soils rather than for use as a fuel. Specific data are presented for commercial lump charcoals and Top-Lit UpDraft (TLUD) charcoals. Easy methods for informal testing of chars are presented to determine several key biochar characteristics. The major conclusions are: 1) Currently available biochars vary significantly in key properties, 2) Great attention should be taken to know the characteristics of any charcoals being added to soils, and 3) Reports of the responses (whether favorable or unfavorable) of plants and soils to biochar applications are of questionable value without corresponding knowledge of the characteristics of the applied biochars.
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