86 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 



that may be held by the soil against the force of gravity. This 

 amount decreases as the height of the column of soil increases, and 

 may also be considerably influenced by the packing of the soil. 

 Since it is almost impossible to treat any soil in the same state of 

 compactness in which it is found in the field, or to establish a stand- 

 ard condition for soils- in vessels, this measure of the water-holding 

 power of a soil is not likely to have precise value. The greatest 

 theoretical objection to it is, that the force tending to remove the 

 water from the soil is of an entirely different magnitude from thai 

 at work as the plant makes its final struggle for water, and that 

 the effect produced by the one force can not serve as a measure of 

 the effect which might be produced by the other. There seems also 

 to be an impression that salts in the soil water operate to raise the 

 wilting coefficient, while decreasing the capillary moisture by lower- 

 ing the surface tension of the liquid. Such an impression arises 

 from the well-known effect of foreign substances on the surface of a 

 liquid. It has been pointed out by Free (121) that salts in solution 

 actually increase the surface tension of the liquid, and this is entirely 

 in keeping with the known properties of solution-. While the pres- 

 ence of solutes may have the effect of weakening the affinity of one 

 water molecule for another, this is fully counterbalanced, in its rela- 

 tion to capillarity, by the greater density of each group of molecules 

 of which the solute forms a nucleus, and the consequent greater 

 affinity between such groups and the solid surface. This affinity i- 

 known by the name of '•capillary attraction." Furthermore, even 

 while admitting that in either the capillary-moisture test or the 

 moisture-equivalent test some of the solutes may he lost with the 

 water which is drained out of the soil, considerable satisfaction is 

 gained from the idea previously set forth that, at the wilting point 

 of soils, these solutes may be absorbed by the colloids. 



It is believed that Hilgard (125) was the first to employ the 

 principle of capillarity for comparing soils. He used a sieve cylin- 

 der only 1 centimeter high, which, after a layer of filter paper was 

 placed in the bottom, was filled level with the soil. This was im- 

 mersed to a depth of 1 millimeter in distilled water, allowed to stand 

 for an hour, and then weighed. The amount of water absorbed, of 

 course, was dependent on the ability of the soil to lift it. a maximum 

 distance of 9 millimeters. 



Briggs and Shantz (114) compared this measure of absorbing 

 capacity with the directly determined wilting coefficients of 15 -oil- 

 whose wilting coefficients ranged form 0.9 to 16.7 per cent. From 

 these comparisons it is evident that a soil which is able to withhold 

 almost no moisture from plants has a fairly high capillarity, but 

 that the latter does not increase in so great a proportion as the 

 wilting coefficient with more retentive soils. Thus it was neces- 



