i 9 i6] SHULL— SOILS 5 



imbibition and absorption are both involved in the starch, and 

 that a much lower value must be obtained than where absorption 

 alone occurs. On the other hand, it will give a higher value than 

 where imbibition alone occurs. 



While these determinations of the surface force in absolutely 

 dry matter are interesting, they have no practical value, for such 

 forces as these do not occur in ordinary soils containing capillary 

 moisture, or even in air-dry soils and seeds, for it is evident that 

 the air-dry soil or seed already holds as hygroscopic moisture the 

 water that it would absorb with such remarkable energy if the 

 particles were absolutely dry. However, the figures give us an 

 idea of the power with which these substances retain the last part 

 of their hygroscopic moisture, which must be a force opposite to 

 and equal to that with which wetting occurs. 



C. Vapor pressure and centrifugal force methods. — 

 Other physical measurements have been worked out, some of which 

 are very useful, as for instance Hilgard's hygroscopic coefficient 

 (16), a measure based on vapor pressure relations, and the moisture 

 equivalent of Briggs and McLane (6). The latter is particularly 

 valuable, since Briggs and Shantz (7) have shown its relation to 

 various physical and physiological amounts of water. But only 

 one of these measurements can be expressed at present in units 

 which permit a comparison of the soil forces with the osmotic 

 forces of the roots of plants. 



physiological measurements 

 The most important attempt at a physiological measurement 

 of the soil forces is that of Briggs and Shantz (8), who use the 

 wilting coefficient, or percentage of moisture in the soil at the wilt- 

 ing of the plant, in determining unavailable moisture. However, 

 recent work by Caldwell (9) and by Shive and Livingston (32) 

 shows that within certain ranges the permanent wilting of the 

 plant is a function of the intensity of atmospheric evaporation, 

 and that the wilting coefficient should be rather a measure of the 

 moisture in the plant at the tune of wilting than of the moisture in 

 the soil. The constancy of this measure is therefore open to some 

 question, and its value and limitations in physiological studies are 

 to be determined. 



