SOIL CONDITIONS AND WATER SUPPLY 59 



rr^, TTT rr moisture equivalent rpi ^ 



constants. Tlius, W. coeff. = _ , /^ ^ — • -^ne 



1-84(1 ± 0*007) 



factors actually found are also shown in Table XIV. The 

 actual factors probably require modification for types of 

 soil of origin widely different from those studied by Briggs, 

 and the relation may not in general be very exactly expressed 

 by such simple equations. As Keen (1922) has pointed out, 

 it would be surprising if relations so intricate were to be 

 susceptible of a simple linear expression. But Briggs 

 results do give us a very striking illustration of the effect of 

 soil constitution on water supply, and they mark an impor- 

 tant step forward in the endeavour to put these relations on 

 a quantitative basis. 



Soil Forces and Water Retention.— We may turn to the 

 problem of what condition in the soil slows down the water 

 supply and occasions wilting in such experiments as those 

 of Briggs, where the soil factors are dominant. The most 

 natural assumption is that at the wilting coefficient the soil 

 water has been reduced to the point when it is present only 

 as extremely fine films, and absorbed by the colloids, and is 

 thus held by the high imbibition forces which amount to 

 many hundreds of atmospheres. 



It is a matter of much difficulty to determine the forces 

 withholding water from the plant at any given degree of 

 moisture. C. A.Shull (19 16) determined the amount of water 

 taken up from soil by the seeds of the cocklebur (Xanthium), 

 which have semipermeable seed-coats. He also determined 

 the amount of water taken up by these seeds from a series 

 of salt solutions of graded osmotic strengths. He argues 

 that the osmotic strength of the solution, from which the 

 same amount of water is absorbed as from a soil of given 

 moisture content, indicates the force with which that soil 

 retains water. Using a heavy silt loam with a wilting 

 coefficient of 19*1 per cent., he obtained the results shown 

 in Table XV. 



