WHAT WATER DOES IN SOILS 49 



ing of the rains, is allowed to remain on the soil surface, 

 it then becomes like the brick; it will draw up the mois- 

 ture from the soil below and waste it by evaporation 

 into the air. 



In the humid region the summer rains often form a 

 crust each time it rains; this crust must then be broken 

 up by cultivation to prevent evaporation. But there 

 the roots of crops are mostly so near the surface that 

 a thick summer mulch cannot safely be made without 

 injuring them. 



Dry farming. To keep up a good mulch during the 

 dry season, and to keep the soil surface open so as to 

 take up quickly any rains that may fall during the wet 

 season, are the main points of what is called "dry 

 farming " (figure 24). By this we mean farming with- 

 out irrigation in regions where the rainfall does not 

 exceed twelve or fourteen inches in a year. Good crops 

 are thus grown where, as in a large part of the higher 

 lands in the arid region, irrigation water is not to be 

 had. Sometimes this is called the Campbell system of 

 farming. J^ Lx/vX^ ^U, 



IRRIGATION 



Of course, what is true of rain is also true of irriga- 

 tion water. When land is flooded, the whole surface 

 is wetted, so that however deeply it may penetrate, 

 there must afterwards be much loss from evaporation, 

 and crusts will form on the surface. 



