RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 137 
physical properties of various soils, but also, for any given soil, ac- 
cording to the manner in which the test is conducted, the age and 
species of the plants employed, and, more important still, according 
to the atmospheric conditions at a time when the moisture supply is 
running low. In short, the wilting coefficient is dependent vary 
largely on the rate at which the plant must obtain water in order to 
balance losses. As the atmospheric conditions are difficult to control, 
and practically impossible to reproduce from time to time and place 
to place, it follows that wilting coefficients are empiric quantities and 
have no precise value. 
7. It is probably very desirable that wilting tests should be con- 
tinued as a further check upon theory, and for the further establish- 
ment of relations between different species and different soils. Rela- 
tive values for different species and soils, of much value and interest, 
are to be obtained through simultaneous tests at any given point, and 
by such comparisons a scale of values either for soils or for species 
may eventually be built up. There are, however, indirect methods of 
arriving at the wilting coefficient which are not only desirable for 
practical purposes, but will add greatly to our understanding of the 
variations in wilting coefficients due to biological and environmental 
factors. 
8. The study of the freezing of soil water, the study of the 
acquirement of moisture by soils when exposed to saturated vapor,, 
and even the behavior of the soil water when subjected to an 
external mechanical force, all point to the fact that water may 
exist in the soil as a liquid, capable of more or less movement from 
one soil particle to another, or as a vapor ; 21 that is, as separate 
water molecules, held in place by the affinity of the solid particles, 
and thereby prevented from moving. All signs, too, point to the 
fact that, except possibly in soils of unusual alkalinity or acidity, 
the soil water is truly nonavailable only when it ceases to function 
as a liquid. While wilting of plants may often occur with liquid 
water still available, this is readily accounted for by the slow rate 
of movement toward the roots, which become a probability, espe- 
cially in clay and humous soils, whenever the volume of water is 
not large. Water obviously moves much more readily in coarse 
than in fine or humous soils; and, as has been mentioned, the rate 
which may be fatal to a plant depends on the needs of the plant as 
determined by its losses. 
9. While, therefore, no method has yet been devised by which the 
theoretical and exact wilting coefficient may be directly arrived at, 
any one of the methods mentioned in the preceding paragraph has its 
21 It would, perhaps, be more descriptive of the kinetic status to speak of this as 
" solidified water " and it is not certain that a wide separation of the molecules, as in 
vapor, is an essential part of the situation. 
