56 ARID AGRICULTURE. 



times be covered with this mulch, which should 

 be in granular form of small lumps. Care should 

 be taken not to get it too' fine. Dust does not 

 absorb moisture so well and is apt to drift or 

 blow away. Working the ground when dry will 

 tend to make the surface soil too fine. The ob- 

 ject of cultivating the summer-fallowed soil is 

 to prevent the formation of a crust after heavy 

 rains ; to allow proper action of sun and air ; to 

 prevent the moisture from rising to the surface 

 where it will evaporate and to destroy tlie growth 

 of weeds. The film moisture in the soil travels 

 up and down by what is called capillary action. 

 This is merely the flowing of the moisture which 

 surrounds one particle of the soil to, and around, 

 the next soil particle, which is close enough to 

 touch the film and so on as long as the soil par- 

 ticles are close enough together, until it reaches 

 the surface where it is evaporated. By stirring 

 the soil, we let in air and separate the grains of 

 soil so the capillary movement is stopped, and 

 loss by evaporation is prevented. The soil mulch 

 must be maintained over the whole field, and 

 the drag harrow is the principal tool used for 

 this purpose. Writers have recommended that 

 those who practiced summer-f nllowing should get 

 on to the land with the harrow immediately after 

 every rain or snow. This is not always neces- 

 sary or advisable, for working too soon may 

 cause rapid loss of the moisture which is still in 

 the surface layer. The ground should be worked 



