PHYSICAL PROPERTIES OF SOILS 



39 



The rainfall was sufficient to have raised the water 

 content of the surface soil to 20.77 P er cent. The sub- 

 soil showed a loss of 1.52 per cent, while the surface 

 soil showed a gain of 1.34 per cent in addition to the 

 water received from the shower. If evaporation begins 

 before the equilibrium is reestablished, there is lost, not 

 only the water from the shower, but also the water 

 which has been translocated from the subsoil to the 

 surface. Hence the importance of shallow surface 

 cultivation immediately after a rain. 



When the subsoil contains a liberal supply of water, 

 and the surface soil a minimum amount, there is after a 

 shower a movement of the subsoil water to the surface. 

 The soil particles at the surface are surrounded with 

 films of water which thicken at the expense of the sub- 

 soil water. Surface tension is the cause of this movement 

 of the water to the surface, and under the conditions 

 stated it is temporarily greater than the force of gravity. 



A hard, thin crust should never be allowed to form 

 after a rain, because it hastens losses by evaporation, 

 while a soil mulch formed by surface cultivation has the 

 opposite effect. 



