HOW MOISTURE IS CONSERVED. 85 



vapor, it becomes again compacted to a degree which re- 

 stores its power to transmit moisture. The cultivator has 

 it, then, within his power to spread a mulch and check 

 evaporation simply by fine and frequent pulverization of 

 the surface layer by cultivation. It is this ability which 

 enables the California horticulturist to transform the 

 lower layers of his soil into a reservoir, and to profit by 

 the natural tendency of the moisture to rise in the com- 

 pact soil until it reaches the point where the pulverized 

 layer checks its advance. This practice makes possible an 

 achievement which seems almost incredible to workers in 

 humid climates, viz. : the growing of a succulent crop 

 from seeding to harvest without the use of a drop of 

 water either by rain or irrigation, and it is this practice, 

 coupled with the deeper rooting habit of plants which is 

 induced by it, which enables our trees and field crops to 

 grow thriftily and produce heavily during months of 

 drought, while a few weeks of drought bring distress to 

 plants in humid climates. 



But the pulverized surface layer must do more than 

 arrest the capillary rise of moisture before it reaches the 

 surface : it must check it at a point out of reach of the free 

 entrance of air to the loose layer, consequently the de- 

 gree of pulverization and the depth of the loose layer are 

 factors to be carefully observed. It is not enough to grind 

 an inch or two of the surface to powder. The free move- 

 ment of air through this shallow layer at least in our sum- 

 mer air with its exceptional thrist, will proceed with evap- 

 oration from the too thinly covered compact portion and 

 the loss of moisture will be only a little less rapid and com- 

 plete than if the surface had not been disturbed at all. 

 The same thing will happen if the surface layer be only 

 coarsely broken to a still greater depth : the passage of 

 air through the clods will be free enough to draw off the 

 moisture and the soil will dry out to a degree which will 

 bring distress to plants which good cultivation would have 

 maintained in vigorous growth. It is plain then that 



