This study aimed at assessing key surface properties of agronomic relevance for products from slow pyrolysis at 250–800 °C, hydrothermal carbonization (HTC), and flash carbonization. The study further aimed at relating surface properties to current characterization indicators. The results suggest that biochar chemical composition can be inferred from volatile matter (VM) and is consistent for corncob and miscanthus feedstocks and for the three tested production methods. High surface area was reached within a narrow temperature range around 600 °C, whereas cation exchange capacity (CEC) peaked at lower temperatures. CEC and pH values of HTC chars differed from those of slow pyrolysis biochars. Neither CEC nor surface area correlated well with VM or atomic ratios. These results suggest that VM and atomic ratios H/C and O/C are good indicators of the degree of carbonization but poor predictors of the agronomic properties of biochar.