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BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTUB 



barest existence, there is a contest for water between two colloidal 

 masses, either of which may be able to absorb 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 



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. 



