BESEAECH METHODS IX STUDY OF FOEEST ENVIEOXMEISTT. 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 Avhole 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. For 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 us 

 50 1"" or 0.50. Similarly, when the moisture content is three times 

 the wilting coefficient, P'=.Z2> atmospheres, the differential is 67 

 atmospheres, and availability is 0.07. It is seen that this is readily 

 expressed by 



M—WC 



giving availability numerical values somewhat proportionate 1<> the 

 osmotic pressures in favor of the, plant. 



i hi. i oi.i i ii ii. .i OS ava [LABILITY. 



As already suggested, tin- ability of the plant to supply itself 

 with water would seem to be measurable in terms of the differential 



