1916] 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 Briccs and McLane (6). The latter is particularly 
valuable, since Briccs 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 Briccs 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 Suive and LivincsTon (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 time 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. 
