Fig. 2 indicates that in our system a 10% error in the value of q results in a nearly constant absolute error in biochar weight of approximately ±0.025 g across the range of tested weights of biochar from 0 to nearly 0.5 g per sample (equivalent to concentrations of biochar from 0 to 10% of soil by weight). A systematic absolute error may be acceptable, particularly for studies in which comparisons among treatments are the major consideration.
Fig. 3 indicates that a 10% error in the value of y results in a variable absolute error in the calculation of biochar weight. At the lowest actual biochar weight (0.05 g, equivalent to 1% biochar concentration) the absolute error is comparatively small (approximately ±0.005 g). At the highest actual biochar weight (close to 0.5 g, amounting to 10% biochar concentration) the absolute error is comparatively large (approximately ±0.05 g). However, the percent error was approximately the same across the entire range of tested weights of biochar; a 10% error in y resulted in an approximately 10% error in the calculation of biochar weight, irrespective of biochar weight. Given our results, it does not seem likely that in the experimental use of biochar in field soils y would change as much as 10%. Thus, the error in y for our test circumstances would appear to be acceptable given the fact that no other analytical method is capable of accurately determining biochar weight. However, our assumptions obviously require validation for each soil and biochar type.
In some soils, 20–25 tonne biochar ha−1 have been applied with beneficial effect (Major et al., 2010a, Major et al., 2010b and Jin et al., 2007). If we assume that the biochar is placed in the top 15 cm of soil and that the soil has a dry specific gravity of 1.2 g cm−3, the concentration of biochar applied at the rate of 25 tonne ha−1 would be 1.4% of soil dry weight, well within the range of this study (0–10%). We conclude that this method, based on the “loss on ignition” methodology, is capable of accurately determining biochar in field soils at realistic concentrations assuming that both y and q are known.