REVIEWS 111 



Shull's object in the investigation here reviewed was to find a 

 method of measuring the force with which the soil particles resist 

 the withdrawal of moisture. Briggs and McLane* had shown that a 

 saturated soil gives up its moisture readily until a state of equilibrium 

 is established between the tension of the water films on the soil par- 

 ticles and an external pull of 1,000 times gravity. The point is known 

 as the "moisture equivalent," and is for all practical purposes a physical 

 constant for any given soil.^ Heavy soils hold large percentages of 

 water at the moisture equivalent ; light soils hold small amounts. Be- 

 low the moisture equivalent the water is held by the soil with increas- 

 ing tenacity. This "back-pull" or water-holding power was known 

 to be very great, and has been the subject of numerous researches. 

 But its magnitude remained almost entirely a matter of conjecture until 

 Shull's work appeared. His investigation is, as he says himself, "a 

 first attempt to throw light on an unexplored region of soil physics." 



Shull's method is simplicity itself. He measures the force with 

 which seeds possessing semi-permeable seed coats absorb water at 

 different moisture contents. Using seeds of Xanthium pennsylvanicum 

 and solutions of sodium chloride, and lithium chloride, he determined 

 the osmotic pressure of this water-absorbing force for different de- 

 grees of moisture between air-dry and slightly over 50 per cent on 

 the basis of the air-dry weight of the seeds. The internal pull of the 

 seeds in their air-dry condition he found to be nearly 1,000 atmos- 

 pheres. The pull decreases rapidly as the moisture content of the seeds 

 increases until about 27 per cent of moisture is reached, then drops 

 more slowly to zero at approximately 53 per cent. Since these data 

 are the basis of his measurements of the moisture-retaining forces of 

 the soil, they are worth giving here (p. 112). 



The next step is simply to pit the internal forces of the seed against 

 the surface forces of the soil particles, instead of against osmotic 

 pressure. When a soil of given moisture content and the Xanthium 

 seeds in contact with this soil reach equilibrium, we know that both 

 the soil and seeds are exerting the same force in holding their mois- 

 ture. As soon as equilibrium is established between the seed and a 

 soil of known moisture content, Shull determines the moisture content 

 of the seed. Knowing the force exerted by Xanthium seeds at differ- 

 ent moisture contents (see table), he knows the force with which 

 the given soil at the given moisture content holds its water. 



■» See note 2, "The Moisture Equivalent of Soils." 



5 For a discussion of the moisture equivalent in relation to other physical proper- 

 ties of the soil, see Free, E. E., loc. cit., pp. 164-176. 



