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SOIL PHYSICS AND MANAGEMENT 



contact, although the finer-grained soil may have many times the 

 moisture content of the other. On the other hand, a clay soil may 

 extract water from a sand soil when in close contact, even if the 

 clay contains several times the amount of water that the sand soil 

 does. The difference will be clue to the fact that in the case of the 

 smaller soil particles in the finer-grained soil the water film will 

 have a greater curvature, and hence will be able to pull water from 

 the sand soil, where the films have less curvature. 



Moisture Equivalent. 2 Briggs and McLane designed a centrif- 

 ugal machine by which moist soils could be subjected to a force of 

 1000 times that of gravity or more. The soils under this condition 

 would lose moisture until the capillary force was in equilibrium 

 with this force of 1000 times gravity, when no further loss would 

 take place. At this point all soils have films of the same thickness, 

 and if the soils are put in close contact there is no tendency for 

 water to move from one soil to the other. The capillary forces are 

 in equilibrium. The per cent of water present at this point is 

 known as the moisture equivalent of the soil. It is always higher 

 than the wilting coefficient and lower than the optimum water con- 

 tent. While not representing any critical moisture content, yet it 

 furnishes a very convenient constant for comparison of different 

 soils. The numerical value of the moisture equivalent depends upon 

 the internal surface of the soil. The following table gives the 

 average for some classes of soils : 



Moisture Equivalents of Some Soil Classes 2 



Since the moisture equivalent bears a rather definite ratio to 

 other soil constants these may be used in its indirect determination. 



Determination of Moisture Equivalents from Other Soil 

 Constants. When the other soil constants, as wilting coefficient, 

 hygroscopic coefficient, moisture holding capacity, or mechanical 

 analysis, are known, the moisture equivalents may be determined 

 indirectly by the following formulae : 



