70 
BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
barest existence, there is a contest for water between two colloidal 
masses, either of which may be able to adsorb the solutes which have 
been in the water about them, and hence eliminate osmotic action in 
the ordinary sense of an interchange between two liquids. Appar- 
ently this contest between the attraction of the soil particles and clay 
masses on the one hand, and the cell walls and protoplasmic masses 
on the other, is not essentially different in principle from osmosis, 
except that in the final stage of the struggle the movement of a 
molecule of water from one side of the line to the other becomes im- 
possible because of the lack of a liquid conductor. It is probably on 
DIAGRAM Z 
ILLUSTRATING THE COMPLEXITY 
OF THE 
WATER- WITH HOLDING FORCES IN SOILS 
AS SHOWN BY THE 
FREEZING BEHAVIOR OF THE WATER 
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this account that, in a strong clay soil, the plant may wilt consider- 
ably before an equilibrium of tensions has actually been produced. 
The important point, however, is that under normal growing con- 
ditions in the plant and soil there is a set of forces at work regulating 
the supply of water to the plant, which is dependent almost wholly 
on the presence of solutes in free water, or osmosis in the ordinary 
sense: while, when the water becomes relatively scarce (this may 
be at 20 or 30 per cent moisture content in a clay soil), an almost 
entirely different set of forces is brought into play. It therefore 
appears that the study of soil moisture is not so elemental as it has 
been supposed, and that the value of soil moisture to the plant can 
not be expressed by a direct linear function of the amount of water 
in the soil. Diagram 2 is inserted to show the nature of the problem. 
Forest investigators must get away from this elemental idea, taking 
up the study of soil moisture at the point to which expert soil physi- 
cists have already brought it. 
