63 



50 percent. 1 



It must not be forgotten however that the 

 entire pore space of the soil is full of air only seldom, 

 if at all. An essential part of normal soils is the 

 system of water films and filaments around and between 

 the soil grains 2 and the space occupied by this capil- 

 lary water system must be deducted from the total pore 

 space in order to determine the space available for air. 

 The amount of water-filled space will vary, according' to 

 the water content of the soil, from nearly 100 percent 

 of the total pore space, in saturated soils, to perhaps 

 five percent of it, or less, in soils which are entire- 

 ly air-dry. Since the amount and arrangement of the 

 capillary water system itself reacts on the arrangement 

 of the solid soil-particles (as noted above), the 

 actual relations between water-filled pore space and 

 air-filled pore space will be exceedingly complex. 



1. Hilgard,- Soils, p. 108 (1906). For mea- 

 surements of percentage of pore space see Boussingault 

 and Lewy,- Ann. chim. phys. 37: 1-50 (1853); ffollny,- 

 Forsch. Geb. Agr. Physik 8: 368-370 (1885); Ramann,- 

 Forsch. Geb. Agr. Physik 11: 303-308 (1888). 



2. See Briggs,- U. S. Bureau of Soils, Bull. 

 10 (1897); Cameron and Gallagher,- U. S. Bureau of 



Soils, Bull. 50 (1907); Free,- Studies in Soil 

 Physics, pp. 3-8, 10-15 (1912). 



