RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 93 



ever, are not given here because the results do not coordinate closely 

 with this set. 



3. It is noteworthy that the coefficients for all species in the lime- 

 stone soil are relatively low, while the really high values are given 

 by the heaviest clay. The latter fact, like the result in a strongly 

 humous soil, is believed to be due to nonconductivity of the clay. 



4. In this case the correlation between wilting coefficients and 

 moisture equivalent is a great deal better than the correlation with 

 capillary moisture. In view of what has been said regarding the 

 level of moisture maintained in each pan, it seems pointed to suggest 

 that the wilting coefficient may depend in some measure on the degree 

 of moisture to which the seedlings have become accustomed. It is 

 only logical to suppose that, if abundant moisture tends to stimulate 

 growth, the seedling may, when drought occurs, be relatively defi- 

 cient in the carbohydrates which assist in osmosis. 



The moisture equivalent is a term devised by Briggs and McLane 

 (113) to define the amount of water held by a soil against a definite 

 external force. In the original experiments of these authors the force 

 employed was a centrifugal force exerting a pull 3,000 times as great 

 as the force of gravity. The small samples of soil were placed in 

 finely perforated cans, which in turn were placed against the inside 

 wall of a heavy cylinder. The latter was caused to rotate rapidly 

 by direct connection with a motor. 



In this early work the writers seem to have made no attempt to 

 correlate the moisture equivalents with wilting coefficients. There 

 was, however, a fairly successful formula devised by which the 

 holding power of the soil was related to the constitution thereof, as 

 shown by mechanical analyses. This, it is believed, has been found 

 of little use. 



It remained for Briggs and Shantz (114) to carry on the wilting 

 tests which showed the real value of the moisture equivalent deter- 

 minations. In these later tests the centrifugal machine was con- 

 siderably improved and its speed automatically controlled, while 

 being cut down to give a pull of 1,000-gravity, since it was found 

 that the higher tension extracted relatively little additional water. 

 A - the result of some hundreds of wilting tests and comparisons with 

 the moisture equivalents of the same soils, it was found that from 

 light sands to the heavier clays a linear relation exists between these 

 two incisures, which is expressed by the formula: 



..... . a. . moisture equivalent 



siting coefficients L84 (1 ±0 .00 T) 



