RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 97 



be remembered that, while the table indicates three times as much 

 water-holding capacity in the granitic loam of high humus content, 

 the actual increase over the same type of soil with little humus is only 

 about 65 per cent on a volume basis. 



The sum of these various effects of different soil properties on the 

 moisture-holding properties is shown in the final line of Table 4, 

 where it is clearly indicated that there is a closer parallelism between 

 wilting coefficients and moisture equivalents than between wilting 

 coefficients and capillarity. Eliminating the granitic gravels, the 

 average variation of the group ratios is only 7.5 per cent from a 

 mean value W. C./M. E. of 0.318. The explanations given, more- 

 over, all tend to confirm the belief that the moisture equivalent 

 obtained with a much greater centrifugal force would give a still 

 closer index to the wilting coefficient of any of these types of soil. 



The hygroscopic coefficient is an expression of the amount of water 

 held by a soil after a limited exposure to saturated water vapor under 

 certain conditions. As in the case of the capillary moisture measure, 

 it appears that Hilgard was the first to make practical use of the 

 absorption powers of soils, to compare them generally as to phsyical 

 properties, and to obtain an approximate measure of their wilting 

 coefficients. More recently Alway (102, 103) has done a large amount 

 of work on this subject, using Hilgard's methods very largely, but 

 also investigating many possible sources of error in the routine 

 treatment of samples. 



It is a very well-known fact that a soil is never entirely devoid of 

 moisture if dried in the air for an indefinite period. On the contrary, 

 if atmospheric conditions did not fluctuate so rapidly there would 

 be at all times an amount of moisture in the soil somewhat propor- 

 tionate to the amount of vapor in the atmosphere. The amount 

 so held is a measure of the soil's hygroscopicity, but not a useful 

 measure because of the changing conditions of the atmosphere. 



Similarly a soil undoubtedly still possesses some hygroscopic mois- 

 ture when dried in an oven at, say, 100° or 110° C. The only way in 

 which the soil can eventually be robbed of all its moisture is by 

 drying in a vacuum, by means of which the constant withdrawal of 

 the atmospheric vapor is assured. For practical purposes, how- 

 ever, drying in an ordinary atmosphere at 110° C. gives a good basis 

 for moisture calculations, since at that temperature the vapor in the 

 atmosphere will be very much rarefied in comparison with its satura- 

 tion capacity. This point is mentioned because it is not infrequently 

 noted, in drying large samples, that they may gain moisture in the 

 hot-air oven if there is a decided increase in atmospheric moisture. 

 To avoid appreciable errors it has been found necessary to avoid 

 final weighings of oven-dried samples on excessively moist days. 

 10163— 22— Bull. 1059 7 



