A general review of gamma-ray attenuation methods used for de-
 termining water content and bulk density of soil is reported in this bul-
 letin. These methods are most applicable to measurements under labora-
 tory conditions, but can also be used for measurements in soil profiles
 under field conditions. Examples are also presented for determinations
 of density of materials other than soil, such as wood from a pine tree
 and geological material such as marble. Theoretical equations which de-
 scribe the attenuation of beam intensity as gamma radiation is trans-
 mitted through soil as well as other materials are presented for both
 single and dual-energy beams of gamma photons. Sensitivity, precision,
 accuracy and experimental errors for the method are evaluated and dis-
 cussed with respect to the theory.


 II. Historical Review of the Development and Use of
 Gamma Radiation Methods
 One of the first successful experiments based upon radiation inter-
 action with matter phenomena for the purpose of measuring bulk density
 was performed by Belcher et al. (2) in 1950. They developed a method
 for measuring water content in soil using neutron moderation and bulk
 density by gamma-ray scattering. Later gamma-ray attenuation methods
 were used for a wide range of purposes: to determine fluid level in
 tanks (80), uniformity of several different materials (3), the density of
 concrete (37), the concentration of solids in fluids (1), the concentra-
 tion of heavy metals in aqueous solution (75 and 85), density and water
 content of pieces of wood (55), and the concentration of tungsten sus-
 pensions (91). These early studies provided important contributions to
 the development and dissemination of radiation methodology in soil
 science.
 Gamma-ray attenuation was first used by Vomocil (90) in 1954 and
later by Bernhard and Chasek (4) to determine bulk density in field
soils. These workers used uncollimated radiation from a 60Co source and
used a Geiger Muller detector to measure radiation intensity. The radia-
tion source was placed inside a vertical tube in the soil and the radiation
detector was placed in a nearby second tube. In situ determinations of
soil bulk density were determined for the soil between the two tubes. In
1957, Van Bavel, Underwood and Ragar (89) used a13Cs as a gamma
ray source and a sodium iodide crystal scintillator detector to determine
soil bulk density under field conditions. Theoretical aspects of the use
of gamma attenuation to determine soil bulk density were investigated
by Korobochkin (52) in 1959 and by Van Bavel (87 and 88) in 1959
and 1960.
 A narrow collimated beam of gamma radiation was first used in lab-
oratory investigations by Gurr (42, 43) and Ferguson and Gardner (25)