The conductivity dilemma: How biochar grain’s chemical composition and morphology hinder the direct measurement of its electrical conductivity

Although electrical conductivity is one of biochar's fundamental properties that affects the way it interacts with microorganisms in soil applications, however its measurement is not univocal. This work starts from the results of a testing campaign on gasification biochar as the basis for a larger discussion on the parameters that affect the conductivity measurement and, therefore, the limits of the most widely used compression measuring approach. Different samples were obtained by combining sifting and grinding on a single batch of biochar. The samples were tested in a PTFE cylinder with conductive plunger and cap in a range of pressures up to 25 bar. The same biochar prepared in different ways showed different conductivity behaviors, with variation up to 85%, proving that the testing condition can strongly affect the measurement. These findings encourage the scientific community to further investigate the meaning of the results obtained with a further recommendation to properly describe the sample preparation and the testing conditions when a compression method is used. The final point of this discussion hypothesizes that neither one of the obtained conductivities truly represents the conductivity perceived by the microorganism that interacts with the char in soil applications.