PHYSIOGRAPHIC FACTORS 



177 



Determination of Volume and Composition.— Total pore space 

 or pore volume (in c.c.) is equivalent to the weight of water (ing.) 

 in the soil at saturation, for water then is assumed to occupy all 

 the space in the soil. Actually not all air can be replaced by water, 

 and the small amount of air remaining at saturation represents 

 what is available to roots regardless of circumstances. Air capacity 

 is the amount of air in soil that has been drained of all gravitational 

 water. It is, therefore, equal to the difference between pore vol- 

 ume and the weight of water at field capacity. Since total water 

 holding capacity and field capacity are constants, it follows that 

 pore volume and air capacity are soil-air constants. The actual air 

 content is not at all constant, for it varies inversely with the water 

 content. Soils with a high air capacity are in general well aerated, 

 but, after prolonged rain or flooding, they may for a time be 

 poorly aerated because water fills so much of their internal space 

 that actual air content is low. 



TABLE 10— Porosity, field capacity, and air capacity of some soils with 

 different textures. After 14 from Kopecky. 



Character of soil 



Compact heavy clay . . . . 



Clay loam 



Compact loam 



Very fine sand 



Friable loam 



Friable fine sandy loam 



Air 

 capacity 



0.4% 



5.0 



6.2 

 10.0 

 12.2 

 14.9 



The composition of soil air may be determined with a portable 

 gas-analysis apparatus. 113 The sample of air can be pumped from 

 the soil through a sampling tube 211 or some similar device, 30 or it 

 can be withdrawn from a unit volume of undisturbed soil. The 

 total percentage of oxygen and carbon dioxide in the soil is usu- 

 ally very nearly that found in air, and, in general, an increase of 

 one results in a proportional decrease of the other. 



