Amount of Rain Needed 121 



very much increase the moisture in the soil above that 

 which falls as rain or snow. 



Further than this, we have no reason to believe 

 that mere tillage, as such, can in any way diminish 

 the rate of transpiration from the crop which is grow- 

 ing upon the soil being tilled, unless, indeed, it should 

 be done by root -pruning, a method decidedly injurious 

 in most cases. It follows, therefore, that in no way 

 can we hope, by methods of tillage, to diminish the 

 loss of water by transpiration through the crop itself. 

 We may, indeed, make the conditions for growth so 

 favorable that the maximum amount of dry matter is 

 developed during the time a given amount of water 

 is being evaporated from the surface of the crop ; but 

 so far as the direct influence of tillage is concerned, it 

 can only lessen the evaporation from the soil surface, 

 and reduce the losses by percolation and by surface 

 drainage. No amount or kind of tillage can dispense 

 with water ; that must be had, either from rain or 

 snow, or be supplied by irrigation. With water enough 

 in the soil to make a crop, good tillage will bring the 

 most out of it ; but when the rainfall has really been 

 deficient, nothing short of irrigation can make the crop. 



AMOUNT OF RAIN NEEDED TO PRODUCE CROPS 

 IN HUMID AND SUB -HUMID REGIONS 



Having pointed out in a general way the limitations of tillage 

 in conserving soil moisture for crop production, it is important to 

 show how great its possibilities may be when unaided by irriga- 

 tion ; for if in humid and sub -humid climates tillage may enable 



