attenuation coefficient is difficult to attain and normally an error of
1.6 to 2.0% is considered experimentally acceptable. The soil hetero-
geneity, the size of its particles and the sample compaction within the
container, are some of the possible sources of errors. For some soils it
is necessary to obtain a large number of determinations with samples of
differing bulk density. A common procedure is to prepare 4 to 8 sam-
ples of different bulk densities and make 6 to 10 determinations for u at
different points along each sample. Using 30 to 100 data points, the
means and the standard deviations can be calculated for t.
 In summary, low experimental values for the mass attenuation co-
efficient, /, indicate poor collimation and detection, and large magnitudes
of the standard deviation may indicate nonuniform bulk density of the
soil sample, chemical heterogeneity of the soil material, insufficient num-
ber of determinations or low intensity for the incident beam of the
gamma radiation.
 Ferraz (26, 27) determined experimental values of mass attenuation
 (Table 4) coefficients of water, wood, geologic materials, and 13 typical
 soils of Sao Paulo State, Brazil. The high values for the first five soils
 were due to the high percentage of iron and clay minerals, and the high
 value for soil sample number 12 was due to high contents of both cal-
 cium and iron. Soils numbered 6 to 10 are sandy in texture with low
 concentrations of Fe, and consequently values of A were low, similar to
 those for the pure sand. The gt value for a soil high in organic matter
 (sample No. 13) was low, similar to that for wood. It is important to
 observe that experimental values for mass attenuation coefficients for
 60 KeV gamma photons differed greatly between soils with different
 chemical compositions relative to the a. values for 662 KeV radiation.
 The high value of standard deviation for soil sample 12 was due to prob-
 lems of nonuniform compaction and heterogeneity, even though the
 number of determinations was twice that for the others.



 VI. Single-Energy Method For Separate Determinations of
 Water Content and Bulk Density of Soil


 A. General Considerations
 When correctly used, the single-energy gamma attenuation method
provides rapid, easy, practical, and precise determinations of soil water
content or bulk density. This method does not need empirical calibra-
tion and offers the advantage of being non-destructive (73). The pre-
cision and accuracy of determinations of either water content or bulk
density is effected by the intensity of the incident radiation, time of
 23