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Fig. 4. Concentrations of propionic, butyric, and iso-butyric acids analyzed by the external calibration and standard addition methods in 3- and 6-mo-old compost samples (mean ± SD; for external calibration n = 14, for standard addition n = 4). Amounts of acids analyzed by the standard addition method are significantly higher than by the external calibration method (* indicates significance at the 0.05 probability level, ** significance at the 0.01 probability level). Only for butyric acid is the difference not significant.
