tions for determinations of soil water content (0) and bulk density (p)
develop which are slightly more complicated than those for the single-
energy method. These equations are presented as equations [Biii] and
[Biv] in Appendix B. For the dual-energy method these two equations
can be used to simultaneously determine both soil water content (0) and
bulk density (p) from measurements of radiation intensity. This method
is particularly useful for determining water content (0) changes in the
surface layers of a soil which swells upon wetting or shrinks upon dry-
ing. For such a soil both the water content and the density undergo
change during cycles of wetting or drying. Use of the dual-energy gamma
attenuation method to simultaneously determine 0 and p is also advan-
tageous for soils that undergo compaction in the surface horizons due to
S the movement of heavy agricultural equipment.
 Americium-241 which emits 60 KeV photons and Cesium-137 which
emits 662 KeV photons are commonly (35, 58, 26) used as radiation
sources for gamma ray attenuation methods of determining 0 and p.


 D. Experimental Errors Associated With Gamma
 Attenuation Methods
 Determinations of soil water content (0) and bulk density (p) have
associated experimental errors. Mathematical equations presented in Ap-
pendix C relate 0 and p determinations to these errors. For the single-
energy gamma attenuation method, a value determined for water con-
tent (0) is primarily attributable to a measured value of the radiation
intensity ratio, I/Io, but improper values used for soil thickness (x),
soil density (p), mass attenuation coefficient for water (g), and mass
attenuation coefficient for the soil (us) will contribute to errors in the
determination of 0. Precision in the values of x, p, g and L have been
shown (36) to be particularly important to determinations of water
content.
 The minimum resolvable change in soil water content, ao, is given
in Appendix C as equation [Cviii]


 "1 e [i I
 a --\Ioexp 2 (Ap 0) [6]

An important observation from this equation is that the minimum re-
solvable change in water content becomes smaller as the unattenuated
radiation intensity (lo) becomes larger. Thus relatively large radiation
S sources are generally used to provide values of Io in the range of 104 to
106 cpm. The value of ac also is influenced by the thickness of the soil
column, the mass attenuation coefficients for soil and water, the density
of the soil, and the soil water content. For most soils the magnitude of