III.] 



WATER RETAINED BY SOIL 



6 9 



Under natural conditions a soil is rarely saturated 

 to the extent indicated in the previous table ; as the 

 rain water enters from above, the surface of the soil 

 is wetted first and the air within the soil finds a diffi- 

 culty in escaping, so that even after long-continued 

 rain the pore space does not become entirely filled 

 with water. 



The following table shows the water contained in 

 a few field soils sampled a day or two after* the cessa- 

 tion of long-continued rain, and calculated as per- 

 centages of the wet soil by weight : — 



Hellriegel has shown that the optimum proportion 

 of water in the soil for the growth of the plant is 

 40 to 50 per cent, of the maximum required for satura- 

 tion. 



