RESEARCH METHODS IN STUDY OF FOREST EXYIKOX.v 



tion of a certain condition of soil moisture, has entifi< 



which fully justifies an elaborate description of them. For examp 

 such methods permit us to compare the drought resistance of eu 

 number of species in any number of soils through any period, pi 

 vided only that the experimental conditions are reproducible. \\ 

 can determine this relative drought resistance, as between two or 

 three species, by wilting them simultaneously in the same soil mas 

 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 n 

 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 l>e 

 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 presupp^c- some 

 knowledge of the plants which may not be available. It must ae< 

 sarily consist of a number of determinations on the same kind 

 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 ol 



availability. 



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



to demonstrate its ability to hold water against the tone oi gravit 



3. The moisture-equivalent determination, in which the mois 



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

 mass. This may be a force created by the centrifugal method one 

 hundred or one thousand times as great as gravity. 



4. The hygroscopic coefficient determination, m which the at 



of the soil for moisture is determined by exposing it to an atmospn* 



of saturated vapor. . , 



The capillary moisture, or '•capillarity," the terms being used 



interchangeably, in this discussion, refers to the quantity ol watei 



