Ash was determined in a variety of plant and organic soil samples by Oxygen—bomb combustion and by a simple muffle—furnace technique. The influence of using different drying temperatures was also assessed. Bomb combustion produced a systematic error of underestimate and a random error of variation between trials. These errors varied among the different types of material tested, but in general both types of error increased with the ash content of the material. The average systematic error of underestimate of ash content for all sampes was 1.46%, which led to an error of 1.56% when adjusting caloric coefficients to an ash—free basis. Lower drying temperatures had an insignificant effect on adjusted caloric coefficients. Independent ash determinations are recommended for materials with ash content greater than 5% to restrict the error of adjusted coefficients below 1%. The mufflefurnace technique is recommended for close approximations of absoulte ash content.