RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 101 
ecologically. Both area and depth of soil which contribute to a given 
plant must be known. 
Availability of the Moisture. 
As already stated, it is intended to confine this term " availability " 
to the simple relation between the whole moisture and the available 
moisture. The term can not be an exact expression of the rate at 
which the plant will be able to obtain water, since such rate depends 
on conditions within the plant as well as those without: or. in brief. 
on the need of the plant for water. It should, however, have greater 
value than a bare measure of whole moisture, or even of available 
moisture in percentage of dry soil weight as an expression of a 
condition of the soil. Its value is predicated on the assumption 
that, at the wilting point, a given plant is probably exerting a fairly 
definite osmotic pressure in its effort to obtain water, and that at 
this time the osmotic pressure of the soil water is also definite and the 
same as that in the plant. This is evidently not the case if the 
wilting coefficient is as low as the point at which both the water and 
solutes become adsorbed by the soil colloids, for at this point the 
osmotic pressure becomes infinitely large. Por this reason the pro- 
posed measure of availability may have only limited usefulness, but 
should at least serve as a stepping stone to the next and more definite 
proposal. 
If. for example, it is assumed that when a plant wilts it is exert- 
ing an osmotic pressure. P. of 100 atmospheres, then supposedly at 
the same time (that is. in the condition expressed by the wilting 
coefficient) the soil is exerting an opposing force. P'. also repre- 
sented by 100 atmospheres. If. then, an amount of water equal to 
the wilting coefficient is added to the soil, the soil solution, roughly 
speaking, has been diluted to one-half its previous strength, and 
there is a differential in favor of the plant of 50 atmospheres. Since 
the starting point was 100 atmospheres, this situation, or the avail- 
ability for this particular plant and soil, may be expressed as 
50/100 or 0.50. Similarly, when the moisture content is three times 
the wilting coefficient. P'=33 atmospheres, the differential is 67 
atmospheres, and availability is 0.67. It is seen that this is readily 
expressed by 
M-WO 
giving availability numerical values somewhat proportionate to the 
osmotic pressures in favor of the plant. 
THE COEFFICIENT OF AVAILABILITY. 
As already suggested, the ability of the plant to supply itself 
with water would seem to be measurable in terms of the differential 
