186 Sowd-Motsture Minimum [384 
THE EXPERIMENTAL DETERMINATION OF A DYNAMIC 
SOIL-MOISTURE MINIMUM 
By Howarp E. PULLING 
The conditions determining the rates of water movement 
in soils have long been recognized as of great importance 
in plant physiology, since they not only limit the amount 
of water a given root system may receive but also modify 
the effects of all soil processes upon rooted plants. In aerated 
soils water is moved by surface forces of the soil-moisture 
films. The magnitude of these forces is dependent upon the 
curvature of the film-air surfaces and not upon the amount 
of water in the soil, so that a soil volume might augment 
its water content at the expense of another contiguous soil 
volume that contained actually less water than the first. 
The amount of water that may be moved in unit time de- 
pends, however, also upon the amount of water in the films 
and a certain minimum should exist below which the quan- 
tity of water subject to capillary movement is too small to 
admit of any but negligible rates, regardless of the mag- 
nitude of the surface forces. 
In aerated soils the water that ‘responds to surface tension 
urge is accumulated around the points of contact of soil 
grains, so that the water adsorbed upon the surface of the 
grains, imbibed by the soil colloids and held as water of 
hydration by the grain constituents need not be considered 
ig the present discussion. It is apparent that the greater 
the number of such points of contact between the soil grains 
in a given gross volume of soil, the greater should be the 
number of similar capillary masses of water and, conse- 
quently, the greater should be the amount of water in the 
soil when the rate of capillary movement becomes negligible. 
Accordingly a complete statement of this minimum for any 
soil may be represented not by a point but by a curve, in 
