232 PLANT SOCIOLOGY 



out of the bath, wiped dry, and weighed. In order to separate the 

 gravitational water from the capillary water both covers are removed 

 and the cylinder containing the soil is placed on a fine sieve. The 

 gravitational water will drip off within 2 hr. After putting on the 

 covers, the weight is taken and the difference gives the air capacity. 

 Drying at 110°C., and reweighing, permits the calculation of the 

 water capacity. 



A modification of this method has been introduced by Siegrist, who 

 intensifies the air replacement by water by applying suction from a 

 vacuum pump. He makes use of cylinders of 250 cc. capacity, as 

 they are more easily manipulated. The cylinders containing the 

 undisturbed soil are put under water in a vessel connected with a 

 vacuum pump. By suction the air contained in the pores of the soil is 

 driven out. The cylinders are closed as in the previous method, 

 weighed, and the separation of capillary from gravitational water is 

 carried out as directed above. Weighing, and drying at 110°C., 

 permits the calculation of both air and water capacity. 



Permeability. — The permeability of the soil is a very important 

 factor in countries which like the Mediterranean region are subject 

 to torrential rains. The denuded clay soil is often not able to retain 

 the water which floods the lower areas of the river basins. These 

 areas of impermeable soils should be kept with their plant cover intact 

 or should be reforested, as a biological defense against erosion. 



