1h Oot COE te re ee ee eee er RN ay cee OR Nene eT Te 
1912] BRIGGS & SHANTZ—WILTING COEFFICIENT 29 
our experiments. A single determination by ALway,”? in which 
barley plants were used, gave a ratio of 0.65. 
In the absence of a more definite relationship between non- 
available moisture and the hygroscopic coefficient, ALWAY™ 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 misleading results may be obtained from this approxi- 
mation, particularly when the moisture supply is limited. 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 non-available for growth, while the second con- 
tains over 5 per cent of available moisture. Simply deducting 
the hygroscopic coefficient would lead to the erroneous conclusion 
that both soils contained considerable available moisture. 
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,™ who, 
in discussing H®EINRICH’s results as given by CAMERON and 
GALLAGHER,*® 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 atmospheric conditions from practically zero in the 
case of some sun-dried desert soils to the hygroscopic coefficient 
™ Atway, F. J., Soil studies i in dry land regions. Bur. Plant Industry, Bull. 130. 
17-42. 1908. 
%3 ALWAY, F. J., Studies of soil moisture in the “Great Plains” region. Jour. 
Agric. Sci. 2:334. 
™ DuccaR, B. Mae Plant physiology. 1911, pp. 56, 57. 
15 CAMERON, F. K., and GALLAGHER, F. E., Bureau of Soils, U.S. Dept. Agric., 
Bull. 50, pp. 57, 58. An error occurs in CAMERON and GALLAGHER’S paper in con- 
nection with Henryricn’s results. They give his determinations on air-dried soils, 
but state that these determinations were made after exposing the soils to a saturated 
atmosphere for a week 
s 
