The wet bulk density of a soil, r, is its mass, including any water present, per unit volume in the ﬁeld; its dry bulk density, rs, is the mass per unit volume of ﬁeld soil after oven-drying. These parameters are related to the soil gravimetric water content, W, as follows:
r r 100 (1) s 100 W
where W is the mass of water expressed as a percentage of the mass of dry soil. The methods available for the measurement of soil bulk density fall into two
groups. In the ﬁrst group are the long-established direct methods, which involve measurement of the sample mass and volume. The mass Ms of the oven-dried sample is obtained by weighing, and the total volume, V, of the soil including air and water is obtained by measurement or indirect estimation. The dry bulk density rs is then given by
Msr (2)s V Such methods have been used by both agricultural soil scientists (Freitag,
1971) and civil engineers (DSIR, 1964), and many of them reduce essentially to the problem of the accurate determination of the sample volume. As these methods have not always proved entirely effective, a second group of methods has evolved in which the attenuation or scattering of nuclear radiation by soil is used to give an indirect measurement of bulk density. Radiation methods are capable of
measuring more accurately and precisely than direct methods, but they too have limitations of their own.