21. De Vries, J. 1969. In situ determination of physical properties of the sur-
 face layer of field soils. Soil Sci. Soc. Amer. Proc. 33:349-353.
22. Durante, V. A,, J. L. Kogan, V. I, Ferronsky, and S. I. Nosal. 1957. Field
 investigations of soil densities and moisture contents. Proc. Fourth Int. Conf.
 .Soil. MNch. Found. Fnc 1 216-220.
 23, Elze...- ., A., R. S M.r.;cll:. :il H.-f. Selim. 1976. Distribution of water
 and herbicide in Lakeland sand during initial stages of infiltration. Soil Sci.
t 122:297-307. .. ---
241.Evans, R. D. 1955. The atomic nucleus. New York, McGraw-Hill.
25. Feuw,.rn. H., and W, H. Gardner. 1962. Water content measurement in
 soil columns by gamma ray absorption. Soil Sci. Soc. Amer. Proc. 26:11-14.
 26. Ferraz, E. S. B. 1974. Determinacao simultanea de densidade e umidade de
 solos por atenuacao de raios gama do 1r7Cs e 241 Am. Thesis, University of
 Sao Paulo, Brazil. 120 p. (in Portuguese).
27. Ferraz, E. S. B. 1976. Determinacao da densida de da madeiras por atenu-
 acao de raios gama de baixa energia. Bol. IPEF, Brazil (accepted for pub-
 lication).
 28. Finnemore, E. J., and J. R. Schaff. 1972. A gamma-ray attenuation system
 for laboratory measurement of soil moisture content. Technical Report No.
 127, Department of Civil Engineering, Stanford University, 100 pages.
 29. Fritton, D. D. 1969. Resolving time, mass absorption coefficient and water
 content with gamma attenuation. Soil Sci. Soc. Amer. Proc. 33:651-655.
 30. Gardner, W. H., and D. Kirkham. 1952. Determination of soil moisture
 by neutron scattering. Soil Sci. 73:391-401.
 31. Gardner, W. H, 1965. Water content. In Methods of Analysis, American
 Society of Agronomy Monograph No. 9, Part I, pp. 82-127.
 32. Gardner, W. H., and M. E. Fischer. 1966. Concurrent measurement of
 bulk density and water content in soil using two gamma-ray energies. Amer,
 Soc. Agron. Abstr., Page 46.
 33. Gardner, W. H., and C. Calissendorff. 1966. Gamma-ray and neutron at-
 tenuation in the measurement of soil bulk density and water content. Int. Publ.
 Wash. State Univ., Wash.
 34. Gardner, W. H., and C. Calissendorf. 1967. Gamma-ray and neutron at-
 tenuation in measurement of soil bulk density and water content. In Sympo-
 sium on the use of isotope and radiation techniques in soil physics and irriga-
 tion studies, Istanbul, 1967, Vienna, International Atomic Energy Association
 Report SM-94/19, pp. 101-113.
 35, Gardner, W. H., G. S. Campbell, and C. Calissendorf. 1969. Water con-
 tent and soil bulk density measured concurrently using two gamma photo
 energies. Oak Ridge, United States Atomic Energy Commission, Report RLO-
 1543-6, 42 pages.
 36. Gardner, W. H., G. S. Campbell, and C. Calissendorf. 1972. Systematic
 and random errors in dual gamma energy soil bulk density and water con-
 tent measurement. Soil Sci. Soc. Amer. Proc. 36:393-398.
 37. Glasheen, R. W. 1954. Automatic control over materials movement. Chem.
SEngr. Progr. 50:487-492.
S. Gopal, S., and B. Sanjeevaiah. 1973. Gamma-ray attenuation coefficient
 Measurements. Phys. Rev. 8(6):2814-2818.
 39. Groenevelt, P. H. 1974. The use of a dual gamma scanner to observe the
 shrinkage of clay. Geoderma 11:287-295.
 40. Groenevelt, P. H., J. G. DeSwart, and J. Cisler. 1969. Water-content mea-
 surement with 60 KeV gamma ray attenuation. Bull. Int. Assoc. Sci. Hydrol.
 14(2):67-77.
 41, Guerrini, I. A. 1976. Difusividade da agua do solo em funcao da umidade
 e do tempo. Thesis, University of Sao Paulo, Brazil 50 p. (in Portuguese).
 42. Gurr, C. G., and T. J. Marshall. 1960. Unsaturated permeability-its
 measurement and its estimation from other properties of the material. Trans.
 Intern, Congr. Soil Sci. (7th) 1:306-309.

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