385] H. E. Pulling 1ST 



which the conditions of the soil-air-water system are repre- 

 sented in terms of any two of the three components. 



For convenience the components soil grains and water may 

 be selected. The number of points of contact in any gross 

 volume are determined by the number, size, shape and ar- 

 rangement of the soil grains. In a sufficiently large volume 

 (a few cubic centimeters, for arable soils) the soil grains 

 may be considered as possessing an average density, size 

 and shape, and this average will not change when other 

 samples of the same soil are considered. Likewise if two 

 samples of the same volume have the same number of soil 

 grains, possessing the same average characteristics, it may 

 be assumed that the average arrangement of grains is the 

 same in each. This will be the more strictly true the longer 

 the grains have existed in those volumes, since the forces 

 of surface tension and gravity will tend to place them in 

 the most stable positions. A relative measure of the proper- 

 ties of a mass of soil grains may thus be obtained for any 

 one soil in terms of its dry weight per unit of gross volume, 

 termed the packing. 



The properties of the water masses in the soil may also be 

 considered as being of average character and since these prop- 

 erties depend upon the shape and size of the water masses, 

 which in turn depend upon the shape and size of the spaces 

 about the points of contact of the soil grains, the number 

 of these points and the amount of water in the soil, they 

 will be sufficiently defined by the amount of water and the 

 amount of soil contained in each unit of gross volume. When 

 these amounts are determined for samples of any given soil, 

 each uniformly packed and permitting only a negligible rate 

 of water movement, the data are at hand for plotting the 

 experimental approximation to the minimum moisture curve 

 between the limits of packing encountered in the samples. 



A method has been devised by the use of which samples 

 of any given soil, each of approximately uniform packing, 

 may be obtained with water contents so small that the rate 

 of water movement is about 0.001 gram in 24 hours through 



