32 The Plant World. 



only four others. The percentages of pore space in these cases 

 have been mathematically computed as 25.90 and 47.64, re- 

 spectively, * and these percentages are independent of the size 

 of particles so long as they are uniform. It should be noted, 

 however that variations in the size of spheres will affect the 

 sizes of the individual interspaces if not their total amount, — 

 a matter which becomes of great importance when permeability 

 to be considered. This simple case is, of course, never realized 

 in nature. Practically all soils consist of materials of many 

 different sizes, and the interspaces between the larger grains 

 are partially filled by the smaller. Did no other factors come 

 into play the effect of this mechanical heterogeneity of soils 

 would be to decrease the amount of pore space in all cases. 

 Another modification of the simple conditions arises through the 

 irregularities in the shape of the soil grains, most of them vary- 

 ing widely from the assumed sphere. This irregularity of shape 

 permits both a more complete interlocking of the grains when 

 the arrangement is as close as possible, and a more open struc- 

 ture when it is as loose as possible; thus permitting the existence 

 of masses of non-spherical particles possessing amounts of 

 pore space greater than 47.64 per cent or less than 25.95 per 

 cent. Other things equal, an increased irregularity in the shape 

 of the particles extends, in both directions, the possible range of 

 percentage of pore space. 



It is apparent that these considerations refer only to the 

 range of texture (taking into account both amount of pore 

 space and size of the spaces) which a certain mechanical compo- 

 sition will permit. They tell us nothing of the actual texture 

 which a given soil will possess under given conditions or concern- 

 ing the texture which will be, so to speak, "normal" to that soil. 

 These are matters of structure, and structure is largely controlled 

 by a constituent not yet discussed — the soil water. The mech- 

 anism is as follows: In a moist, but unsaturated soil the water 

 exists mainly in the form of thin films about the soil particles 

 and, where particles touch, the films are also in contact. J At 

 each point of contact the water films fuse, filling the annular 

 space between the particles and forming a water mass of wedge 

 shaped cross-section and bounded by an inwardh curved water- 



"*Slichter, — U. S. Geol. Surv., Ann. Rep. 19. II: 306 et seq. (1899.) 

 JSee Briggs, Bull. 10. Bureau of Soils U. S. Dcpt. of Agr. 0807). 



