66 WILTTXG COEFFICIENT FOR DIFFERENT PLANTS. 



ratio obtained in our experiments. A single determination by 

 Alwa}~ x in which barley plants were used gave a ratio of 0.65. 



In the absence of a more definite relationship between the wilting 

 coefficient and the hygroscopic coefficient, Alway 2 has advocated 

 deducting the hygroscopic coefficient from the field soil-moisture 

 determinations as a basis for comparing the available moisture in 

 soils. Our measurements, however, show that the wilting coefficient 

 is about 1.47 times the hygroscopic coefficient, so that very mis- 

 leading results may be obtained from this approximation, particu- 

 larly when the moisture supply is hmited. For example, consider 

 two soils containing, respectively, 14.7 per cent and 20 per cent of 

 water and each having a hygroscopic coefficient of 10 per cent. 

 Under these conditions all the water in the first soil is practically 

 nonavailable for growth, while the second contains over 5 per cent 

 of available moisture. Simply deducting the hygroscopic coefficient 

 would lead to the erroneous conclusion that both soils contained 

 considerable moisture available for growth. 



It is important in this connection to distinguish clearly between 

 the hygroscopic coefficient, as used above, and the hygroscopic 

 water content, which is simply the water content of "air-dry" soil. 

 The latter term has recently been used by Duggar 3 who, in discuss- 

 ing Heinrich's results as given by Cameron and Gallagher, 4 says: 



It will be noticed that so soon as the amount of water in ordinary soil becomes 

 about three times the hygroscopic water content, it begins to assume physiological 

 importance. 



The water content of air-dried soil may vary according to atmos- 

 pheric conditions from practically zero in the case of some sun- 

 dried desert soils to the hygroscopic coefficient when exposed in a 

 saturated atmosphere. There is, consequently, nothing definite or 

 reproducible about such determinations, unless the conditions under 

 which the measurements were made are also known, and any ratio 

 derived from such measurements is likely to give misleading results 

 when applied to other determinations. 



RELATION" OF THE WILTING COEFFICIENT TO THE SATURATION COEFFICIENT AND TO THE 

 MOISTURE-HOLDING CAPACITY OF SOLLS. 



The saturation water content or the saturation coefficient is the 

 percentage of water held in the soil when all interstitial space is filled 



i Alway, F. J. Some Soil Studies in Dry-Land Regions. Bulletin 130 (Dry-Land Agriculture), Bureau 

 of Plant industry, U. S. Dept. of Agriculture, 1908, pp. 17-12. 



2 Alway, F. J. Studies of Soil Moisture in the Great Plains Region. Journal of Agricultural Science, 

 vol. 2, 1908, p. 334. 



3 Duggar, B. M. Plant Physiology, 1911, pp. 56-57. 



* Cameron, F. K., and Gallagher, F. E. Bulletin 50, Bureau of Soils. TJ. S. Dept. of Agriculture, pp. 

 57-58. An error occurs in Cameron and Gallagher's paper in connection with Heinrich's results. They 

 give his determinations on air-dried soils, but state that these determinations were made after exposing 

 tLs soils to a saturated atmosphere for a week. 

 230 



