EESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 85 
tion of a certain condition of soil moisture, has a scientific value 
which fully justifies an elaborate description of them. For example, 
such methods permit us to compare the drought resistance of any 
number of species in any number of soils through any period, pro- 
vided only that the experimental conditions are reproducible. We 
can determine this relative drought resistance, as between two or 
three species, by wilting them simultaneously in the same soil mass, 
and gradually, by one comparison and another, include all of our 
species and all of our soils. Even this method, however, is not free 
from the necessity for uniform conditions in the successive tests. It 
is therefore best that each wilting coefficient, while being determined 
under some arbitrary and standard set of conditions, should be re- 
lated to some other measure of the soils' water-holding capacity 
which, under reproducible test conditions, always means just one 
thing. In this way an enormous number of comparisons may be 
made between the wilting coefficients for different soils and different 
species. Such physical determinations may also lead to a critical ex- 
amination of wilting coefficients and to the most desirable standard 
methods for their determination. 
Of the various indirect methods which have been devised may be 
mentioned : 
1. The determination of the antiosmotic pressure of the soil, corre- 
sponding to the maximum osmotic pressure which the species under 
consideration is known to tolerate without fatal results. This method 
is obviously not so useful as the others, since it presupposes some 
knowledge of the plants which may not be available. It must neces- 
sarily consist of a number of determinations on the same kind of 
soil, at different moisture contents, until the moisture condition is 
found at which the freezing point becomes " submerged ; " that is, 
becomes indeterminate. Obviously, this leads' to the region in which 
the freezing-point determinations are least precise. While not 
abandoned, this method will be laid aside to be discussed more fully, 
and in its most useful aspects, in connection with the coefficient of 
availability. 
2. The capillary moisture determination, in which the soil is allowed 
to demonstrate its ability to hold water against the force of gravity. 
3. The moisture-equivalent determination, in which the moisture 
in the soilis subjected to any definite force, dependent on its own 
mass. This may be a force one hundred or one thousand times as 
great as gravity created by the centrifugal method. 
4. The hygroscopic coefficient determination, in which the affinity 
of the soil for moisture is determined by exposing it to an atmosphere 
of saturated vapor. 
The capillary moisture, or " capillarity,' 7 the terms being used 
interchangeably, in this discussion, refers to the quantity of water 
