220 Bulletin 37. 



the cultivation that must follow each irrigation, in order to check 

 evaporation and admit air, is obviated by applying the water dur- 

 ing the winter, instead of during the summer. 



Reasons for Cultivation and Weed Destruction. 



After each summer shower the surface of the soil should be 

 cultivated to break up the crust that forms, and thus check evap- 

 oration and permit the entrance of air. The conservation of moist- 

 ure 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 capillary 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 preventing 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 on 

 only in a closely packed soil furnishing 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 cannot pass beyond it by capillary action, 

 and evaporation thus proceeds much more slowly than if the moist- 

 ure were permitted to follow the capillary tubes to the surface. 



vS a m pies of soil taken in an orchard during the summer of 

 1900 illustrate the foregoing. Most of the orchard had been thor- 

 oughly cultivated ; but a portion had been left uncultivated, and 

 had become overgrown with weeds. A determination of the per 

 cent of water in each of the five upper feet in each area May 23d 

 showed that as a whole the upper five feet of soil in the cultivated 

 area contained over a third more water than the upper five feet in 

 the uncultivated area. But when only the available water in each 

 is taken into consideration, the difference is much greater. Plants 

 cannot remove all the water a soil contains. In such a soil as the 

 above, at least five per cent would be left in it after the rootlets 

 had removed all they had power to remove. Making this deduc- 



