the minimum resolvable change in water content ranges from 0.002 to
0.005 cm3/cm3.
 The minimum resolvable change in soil bulk density, ap, is also given
in Appendix C as equation [Cxv]

 a / exp (,p + 1,0) [7]

which differs from equation [6] only in that g, rather than ~t, appears
in the denominator. The magnitude of the minimum resolvable change
in bulk density ranges from 0.10 to 0.20 g/cm3 for most soils.
 For dual-energy gamma ray attenuation methods, errors in concur-
rent determinations of soil water content 0 and bulk density p have been
shown (36) to result from the random emission of gamma rays from the
sources, random errors in mass attenuation coefficients, and resolution
time of the gamma photon counting system. For counts greater than 106
cpm measured in air from 241Am and 137Cs sources, Gardner et al. (36)
stated that the standard deviation in both water content and bulk den-
sity determinations becomes limited by the precision of measurement of
soil and water attenuation coefficients and of soil column thickness.
 An important observation is that the errors in bulk density or in water
content for measurements with the dual-energy method are nearly always
greater (see Appendix C) than the errors when the single-energy method
is used. In the single-energy method, the error in determination of p
does not influence the determination of 0, since p is taken to be constant.
However, for the dual-energy method, errors in the determination of p
increase errors in 0 determinations (see equations [Cxviii] and [Cxix]
in Appendix C). Therefore the dual-energy method should only be used
under conditions where the soil bulk density changes with the water
content, e.g. soils with montmorillonitic clay minerals which undergo
swelling upon wetting and shrinking upon drying or where the bulk
density of the surface soil changes with time due to compaction by agri-
cultural equipment. Under more frigid winter climates than normally
occur in Florida, the bulk density of the surface soil may also change
due to freezing and thawing of soil water. Thus, the dual-energy gamma
attenuation method could also be used under conditions of freezing and
thawing of the soil.


 E. Optimum Thickness of the Soil Column
 The best results for determining soil water content using equation
 [Aiv] in Appendix A, will be obtained when the value of the minimum
 resolvable change in water content (ae) takes the smallest possible value.
 A convenient means to minimize the value of as is to optimize the
 thickness of the soil column. Equation [Cviii] in Appendix C expresses