KESEAECH METHODS IX 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 

 ;il»;indoned, this method will be laid aside to be discussed more fully, 

 and in its most useful aspects, in connection with the coefficient of 

 availability. 



i'. The capillary moisture determination, in which the soil is allowed 

 to demonstrate its ability to hold water against the force of gravity. 

 '■',. The moisture-equivalent determination, in which the moisture 

 in the soil is subjected to any definite force, dependent on its own 

 mass. This may be a force one hundred or one thousand times as 

 great ae gravity created by the centrifugal method. 



4. The hygroscopic coefficient determination, in which the affinity 

 of the -oil for moisture is determined by exposing it to an :it mosphere 

 of -ni urated \ ■> por. 



The capillary moisture, <»r "capillarity," the terms being used 

 nterchangeably, m thie discussion, refers to the quantity of water 



