214 
Journal of Agricultural Research 
Vol. VIII, No. 6 
exceedingly slow rate. If the intensity of the atmospheric evaporation 
is greater than the velocity with which the water is extracted from the 
soil and absorbed, the plants will wilt. The wilting of plants, therefore, 
appears to be due mainly to two factors: (i) To the large force with 
which the unfree water is held in the soil, and (2) to the great osmotic 
pressure of the soil solution at and below the wilting coefficient. As has 
been shown in other publications (2), the osmotic pressure of the soil 
solution of the complex types of soil slightly above the wilting coefficient 
amounts to 15 atmospheres, and its magnitude increases tremendously 
with small decreases in the moisture content. 
The foregoing views that the soil moisture at the lower magnitudes of 
water content existing in the solid phase is due to the chemical and 
physical absorbing forces, and that the wilting of plants is due mainly 
to the great force with which this water is held in the soil and also to the 
large osmotic pressure of the soil solution, are contrary to the prevalent 
views upon the subjects. In the first place, it is quite generally believed 
that the water in the soil at the lower magnitude of moisture content 
and even below the hygroscopic coefficient exists as films in the liquid 
phase. That this is the common view is indicated by the following facts: 
(1) The movement of moisture is attributed to the curvature of the 
capillary films. (2) Attempts have been made to measure the thickness 
of these films around the soil particles or Saccules; the crumb structure 
in the soil is attributed to the force of the water films, etc. 
In the second place, the wilting of plants is attributed by some investi¬ 
gators mainly to the slow movement of the moisture in the soil. This 
view is well expressed by Shull (9) in a recent article. He says (p. 28): 
The view is held, therefore, that the wilting at this critical soil moisture content 
must be due to the increasing slowness of the water movement from soil particle to 
soil particle, and from these to the root hairs, the rate of movement falling below that 
necessary to maintain turgidity of the cells of the aerial parts, even under conditions 
of low transpiration. 
Shull came to this conclusion from a comparison between the force 
with which the moisture is held in the soil at the wilting coefficient and 
the osmotic pressure of the sap or root cells. He measured the force 
with which the soils retain moisture by means of Xanthium seeds. He 
found that at the wilting coefficient this force amounts to only four 
atmospheres. The osmotic pressure of the sap of root cells amounts to 
about seven or eight atmospheres or twice as much as the force of the soil. 
From these data he logically concluded, therefore, that at the wilting 
coefficient there is still plenty of moisture and a gradient for the move¬ 
ment of water toward the plant, and yet the plants wilt; this wilting, 
therefore, is due to the slow movement of the soil moisture. 
The magnitude of the force with which the moisture is held in the soil 
at the wilting coefficient, according to the method of Shull, is not con¬ 
firmed by the results obtained by the freezing-point method and the 
