34 SOILS AND FERTILIZERS 



face 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 a subsoil contains a liberal supply of water, 

 and the surface soil a minimum amount, there is after 

 an ordinary 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 subsoil 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 thin hard crust should never be allowed to form 

 after a rain, because it hastens the losses by evapora- 

 tion, while a soil mulch formed bv surface cultivation 

 has the opposite effect. 



28. Rolling. — The use of heavy rollers for com- 

 pacting the soil is beneficial in a dry season on a soil 

 containing large proportions of sand and silt. Rolling 

 the land compacts the soil and improves the capillary 

 conditions, enabling more of the subsoil water to be 

 brought to the surface. Experiments have shown 

 that when land is rolled the amount of water in the 

 surface soil is increased. This increase is, however, 



