94 NATURE AND PROPERTIES OF SOILS 



soils are heavy due to the close contact of the particles, while 



the finer soils are much lighter due to the tendency of the 

 small particles to resist compaction,^ This means that soils 

 such as sands and sandy loams contain less pore space than 

 silt loams, clay loams, and clays. "While the heavier soils have 

 more combined air and water space, the individual spaces are 

 much smaller than in the sands, which accounts for the slow 

 air and water drainage in the former and the ease with which 

 such phenomena take place in the lighter soils. 



A very simple formula may be used to calculate pore space, 

 providing the specific gravity and volume weight are known. 

 It is subject to considerable inaccuracy, however, because 

 of the presence of colloidal matter, the exact influence of which 

 cannot be determined. 



^ ^ c, ^^^ /vol. wt. 100\* 



% Pore Space = 100 — I x -^r- I 



^ ^ \sp. gr. 1 / 



A soil having a volume weight of 1.6 and a specific gravity 

 of 2.6 has, according to this formula, 38.5 per cent, of pore 

 space. A soil in which the above figures are 1.1 and 2.5, 

 respectively, possesses 56 per cent, of air and water space. 



The following figures taken from King ^ illustrate the rela- 

 tion that texture and, to a certain extent, structure also occu- 

 pies in relation to soil pore space : 



^ Sandy soils are generally spoken of as loose, wMk elays aie called 

 compact. The term compact is thus used in the sense of hard, unyielding, 

 stiff, or impenetrable, and does not indicate that the pore space of clay 

 is less than that of a sandy soil. 



^It has already been explained in a previous footnote (see under 

 volume weight) that the specific gravity of a soil represents 100 per 

 cent, soil material or the weight of absolutely solid soil. Volume weight 

 indicates in comparison thereto, the soil material actually present. The 

 ration of the specific gravity to the volume weight when multiplied by 

 100 becomes the percentage of the soil volume occupied by the soil 

 particles. 



® King, F. H., Fhysics of Agriculture; published by the author, Madi- 

 son, Wisconsin, 1910. 



