138 BULLETIN" 1059, U. S. DEPARTMENT OF AGRICULTURE. 



possibilities as an avenue of approach. Thus, by the freezing-point 

 method the osmotic pressure of the soil solution corresponding to 

 any given moisture content may be determined; and by several 

 trials, the point at which the soil water ceases to behave as a liquid, 

 that is, ceases to show a definite freezing point, may be determined 

 quite closely. This water content is probably the term sought as a 

 standard. The freezing-point method has one serious objection: 

 namely, that it forbids keeping the soil during test in a natural state 

 of compactness, or with a natural arrangement of the soil particles. 



The vapor transfer method has also many possibilities. In the 

 ordinary determination of the hygroscopic coefficient the watei 

 vapor is probably not entirely saturated; and, in consequence, under 

 certain empiric conditions of the test, there may be obtained in a 

 limited time a limited absorption of vapor by the soil which was 

 air-dry at the outset. The quantity absorbed, so far as the tests go 

 bears a fairly constant relation to the wilting coefficient, the ratio of 

 the two quantities being approximately 2:3. The objection to this 

 method is that the conditions are purely empiric, and the quantity 

 of soil treated is very small. 



By exposing soils to water vapor in a vapor-tight chamber, such 

 as a bell jar, and in the presence of a solution of an active salt which 

 represents a given osmotic pressure and vapor pressure, considerable 

 masses of soil may, after a long period, be brought to vapor pres- 

 sure equilibrium with the solution and with one another. The mois- 

 ture content of each soil is then the " osmotic equivalent " of the con- 

 trol solution, whose osmotic pressure is readily calculated. The 

 osmotic pressure to exist at the end of the test may be in part con- 

 trolled by calculations at the outset, and later by changing the con- 

 trol solution. A solution, at the end of the test, representing 50 

 atmospheres osmotic pressure, for example, might be taken as a 

 standard for establishing osmotic equivalents in lieu of wilting coeffi- 

 cients. The objections to this method are the long time required to 

 complete a test and the absolute need for a constant temperature, or 

 at least for the elimination of rapid changes. The former objection 

 is partly counterbalanced by the number of soils which may be 

 treated simultaneously. 



In the moisture-equivalent determination, as so far conducted, the 

 moisture of any soil is submitted to a definite centrifugal force which 

 tends to separate it from the soil. Within the limits of agricultural 

 types of soil, at least, the force of 1,000 gravity employed by Briggs 

 and Shantz (114) appears to leave in the soils amounts of water 

 which bear a nearly constant relation to the respective wilting co- 

 efficients. Experiments with a force of 100 gravity have shown 

 wide variations in results with different types of forest soils, indicat- 

 ing that, as humus and clay proportions vary, both strong and weak 



