Bulletin No. 35. 124 



produce much better results than two or more light irrigations 

 with or without intervening cultivation. In all cases, the irrigat- 

 ing stream should be prevented from flowing over the soil about 

 the plants. 



CULTIVATION. 



The effects of the cultivation of growing vegetables are ( 1 ) 

 the aeration of the soil, (2) the conservation of moisture, and (3) 

 the destruction of weeds. 



The aeration of the soil is very important. That the necees- 

 sary biological and chemical processes may proceed properly in 

 the soil, a constant supply of oxygen is essential. If these proces- 

 ses cannot continue, a crop may starve, though there be an abund- 

 ance of raw material in the soil. After rains and more especially 

 after irrigation, most soils form a crust over the surface, or "bake" 

 to some depth, and free acess of air is thus prevented. Cultiva- 

 tion breaks up the surface and promotes the aeration of the under- 

 lying soil. 



The conservation of moisture by cultivation is based on well- 

 established principles. During a rainstorm or during irrigation, 

 the water received by the soil moves downward. As soon as the 

 supply from above ceases and the free water settles away, by capil- 

 lary action the movement of the moisture in the soil sets in in the 

 opposite direction, moving upward as well as downward. As the 

 moisture reaches the surface, it passes off as vapor. Only by pre- 

 venting the water reaching the surface can this evaporation be 

 checked. The capillary action by which the water reaches the 

 point where it evaporates can go only in closely packed soil fur- 

 nishing the innumerable, minute, irregular tubes through which 

 the water rises. To break up these tubes checks this upward 

 movement. Cultivation not only breaks up the capillary tubes of 

 the surface, but forms over the surface a mulch that prevents rapid 

 evaporation. The moisture will then rise to the mulch, but can- 

 not pass beyond it by capillary action, and evaporation thus pro- 

 ceeds much more slowly than if the moisture were permitted to 

 follow the capillary tubes to the surface. 



In order to produce the best results the soil must be so cul- 



