RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 137 



physical properties of various soils, but also, for any given soil, ac- 

 cording to the manner in which the test is conducted, the age and 

 species of the plants employed, and, more important still, according 

 to the atmospheric conditions at a time when the moisture supply is 

 running low. In short, the wilting coefficient is dependent very 

 largely on the rate at which the plant must obtain water in order to 

 balance losses. As the atmospheric conditions are difficult to control, 

 and practically impossible to reproduce from time to time and place 

 to place, it follows that wilting coefficients are empiric quantities 

 and have no precise value. 



7. It is probably very desirable that wilting tests should be con- 

 tinued as a further check upon theory, and for the further establish- 

 ment of relations between different species and different soils. Rela- 

 tive values for different species and soils, of much value and interest, 

 are to be obtained through simultaneous tests at any given point, and 

 by such comparisons a scale of values either for soils or for species 

 may eventually be built up. There are, hoAvever, indirect methods of 

 arriving at the wilting coefficient which are not only desirable for 

 practical purposes, but will add greatly to our understanding of the 

 variations in wilting coefficients due to biological and environmental 

 factors. 



8. The study of the freezing of soil water, the study of the ac- 

 quirement of moisture by soils when exposed to saturated vapor, 

 and even the behavior of the soil water when subjected to an ex- 

 ternal mechanical force, all point to the fact that water may exist 

 in the soil as a liquid, capable of more or less movement from one 

 soil particle to another, or as a vapor; 21 that is, as separate water 

 molecules, held in place by the affinity of the solid particles, and 

 thereby prevented from moving. All signs, too, point to the fact 

 that, except possibly in soils of unusual alkalinity or acidity, the 

 soil water is truly nonavailable only when it ceases to function 

 as a liquid. While wilting of plants may often occur with liquid 

 water still available, this is readily accounted for by the slow rate 

 of movement toward the roots, which becomes a probability, espe- 

 cially in clay and humous soils, whenever the volume of water is 

 not large. Water obviously moves much more readily in coarse 

 than in fine or humous soils; and, as has been mentioned, the rate 

 which may be fatal to a plant depends on the needs of the plant 

 as determined bv its losses. 



9. While, therefore, no method has yet been devised by which the 

 theoretical and exact wilting coefficient may be directly arrived at, 

 any one of the methods mentioned in the preceding paragraph has its 



21 It would, perhaps, be more descriptive of the kinetic status to speak of this as "solidified water" and 

 it is not certain that a wide separation of the molecules, as in vapor, is an essential part of the situation. 



