EDAPHIC OR SOIL FACTORS: PHYSICAL 231 



consists of strong main and tributary axes but only short, vertical 

 feeding roots, rarely exceeding 15 cm. in length. 



Determination of Air Capacity. — The pore volume of a soil does not 

 give a reliable measure of its aeration. Burger (1922, p. 185) has 

 shown that even a soil with a pore volume of 70 per cent may be 

 poorly aerated in the saturated condition. It is for this reason that 



Fig. 118. — An association of Salicornia macrostachya on a saline soil of piilygonal 

 structure; aestival aspect. Etang de Vic, France. (Photo by Braun-Blanquet and 

 Keller.) 



the determination of air capacity is recommended, not only because 

 of its great practical significance {e.g., as an important criterion for 

 the estimation of the quahty of a forest soil) but also because an 

 approximate method, proposed by Wiegner, furnishes useful results 

 without much trouble. 



Soil samples collected in undisturbed condition in steel cylinders of 

 1,000 cc. volume, which are closed at one end with a tightly fitting 

 steel cover, are put into a basin of water and left there until completely 

 saturated with water. After 24 hr. the cylinders are closed under 

 water by putting on the second steel cover. The cylinders are takea 



