When calculating the mean weight-diameter (MWD) or the geometric mean diameter (GMD) of a soil sample, the arithmetic mean diameter (in the case of MWD) or the logarithmic mean (in the case of GMD) of cach class is weighed by the mass of aggregates retained in that class. The use of the arithmetic mean of each class, multiplied by the aggregate mass retained in that class is correct only if a symmetric distribution of aggregate sizes occurs within each class, which is very unlikely. In this paper, developed by the end of 1996 at the Soil Science Department of the UFRGS, Porto Alegre, Brazil, a new method is proposed aiming the calculation of the mean diameter of aggregates of soil samples, assuming a similar distribution of aggregate sizes within each class and within the whole sample. It is assumed that the mean diameter of each class is better estimated by that diameter that divides the aggregates contained in the class in two parts with equal mass. A calculation procedure is presented and the calculated mean diameter is referred to as "mean adjusted diameter" (MAD). To evaluate the method, it is applied on existing data from different soils, management conditions and cropping rotations. It is concluded that the MAD should be used as a substitute of the MWD and GMD, as it represents the mean aggregate diameter of a soil sample in a more correct way.
mean weight-diameter; geometric mean diameter; mean adjusted diameter; aggregate stability