164 Irrigation and Drainage 



8. Frequency of Tillage to Conserve Soil Moisture 



Tillage to conserve soil moisture, like water for irrigation, 

 cannot be applied except at an increased cost of production. 

 Hence, to cultivate a field when there is nothing to be gained 

 from it is to be avoided. In the early part of the growing sea- 

 son, when the soil is so fully charged with moisture that a small 

 rain easily causes the soil granules to coalesce and destroy the 

 effectiveness of mulches, it is often desirable to repeat the culti- 

 vation or harrowing as often as there has been a shower of suffi- 

 cient intensity to establish good capillary connection between 

 the stirred and unstirred soil. 



It is often of the greatest importance that this reestablish - 

 ment of the mulch should take place at the earliest possible 

 moment, not only because of the rapid loss of water from wet 

 surfaces, but because of the fact that, when the surface soil has 

 reached a certain degree of dryness while the deeper soil is yet 

 wet, the moisture of the surface layer so strengthens the upward 

 movement of soil moisture into that layer that not only is all 

 of the rain held at the surface, but a very considerable amount 

 of the deeper soil water is brought there also. Our studies have 

 proved, both by observation and by repeated experiment, that 

 wetting the surface of the ground may leave the deeper soil 

 actually dryer than it was before, and if the new mulch is not 

 early developed the rain may leave the surface four feet dryer 

 than it would have been had the rain not occurred. 



Then, too, in the early part of the year, there are so many 

 advantages to be gained through frequent stirring of the soil, 

 other than the saving of moisture, that the slightest reason for 

 going over the ground again should lead to its being done. But 

 as the season advances, and the soil has become dryer to con- 

 siderable depths, then the desirability of frequent stirrings of 

 the surface to develop or restore the texture of the mulch, is 

 much less. This is so, partly because when the surface of the 

 ground is dry, it is an excellent mulch, even though it is quite 

 firm and close in texture ; but also, because the smaller showers 



