Relation of Nonavailable Water to Hygroscopic Coefficient 15 
as thus determined — which is much below the hygroscopic co- 
efficient as determined by the Hilgard method. 
Mitscherlich 1 in an exhaustive study of the relation of the 
relative hygroscopicity of soils to their other properties con- 
cludes that a knowledge of the hygroscopicity of soils is of ex- 
treme importance. He holds that the determination by Hilgard's 
method gives results much too high on account of the condensa- 
tion of moisture on the exposed samples. From theoretical con- 
siderations he concludes that soils absorb their maximum amount 
of hygroscopic moisture when allowed to come into equilibrium 
with an atmosphere in contact with a 10 per cent sulphuric acid 
solution. 
Hedgcock 2 experimenting with a great variety of plants on 
six different soils concluded that the ability of plants to take 
water from the soil varies in an ascending scale from hydro- 
phytes, thru mesophytes, to xerophytes, there being variations not 
only among genera but also among species and among individ- 
uals. He reports no determination of the relative hygroscopicity 
of the soils used. 
Clements 3 has proposed the use of the terms "chresard" and 
"echard" for the available, or physiological, water content and 
the nonavailable water content of the soil respectively. "All soils 
contain more water than can be absorbed by the plants which 
grow in them. This residual water which is not available 
for use, varies for different soils. * * * It differs, but to a 
much less degree from one species to another. A plant of xero- 
phytic tendency is naturally able to remove more water from 
the same soil than one of mesophytic or hydrophytic character. 
* * * After one has determined the physiological water for 
the great groups of soils, it is more or less possible to estimate 
the amounts in the various types of each." He describes in de- 
tail the method of determining the "echard" by pot experiments. 
Later 4 he states: "This available water, or chresard, differs for 
the different soils, and, for dissimilar species of plants." No 
reference is made in either of these two books to the hygroscopic 
coefficient of soils or to any other means of determining the 
"echard" of a soil than by the physiological method. "The 
amount of water that a plant can absorb from the soil can be 
readily determined only by finding the amount left when the 
plant wilts completely." 5 
1 Mitscherlich, A. Bodenkunde fur Land- und Forstwirte, Berlin, 1905. 
2 Hedgcock, G. G. Botanical Survey of Nebraska, vol. 6, 1902. 
3 Clements, F. E. Research Methods in Ecology, Lincoln, Nebr., 1905, 
pp. 5 and 31. 
4 Clements, F. E. Plant Physiology and Ecologv, New York, 1907, p. 9. 
6 Ibid., p. 1 3. 
