Tillage to Conserve Moisture 129 



intertillage and surface tillage generally can be counted upon as 

 capable of saving to the crop which is to be grown upon the 

 ground only a part of the rains which fall in winter and spring. 

 The rains of later June and July, August and September are 

 usually beyond the power of tillage to conserve in any marked 

 degree, without at the same time seriously injuring the roots of 

 vegetation growing upon the ground. 



In the first place, after the last of June, in climates like 

 that of the thirteen states selected, the water of nearly all rains 

 is absorbed and retained in the surface 3 inches of soil or less. 

 It is only the rains exceeding 1 inch which penetrate more deeply 

 than this ; and to stir a wet soil is to hasten the rate of evapora- 

 tion of moisture from the soil stirred. If, then, the roots of a 

 crop have dried the surface 8 inches of soil so that it contains 

 but 20 to 30 per cent of its full amount, and a rain falls which 

 wets in but 2 inches, stirring that soil can save but little of the 

 moisture. Further than this, when the surface of the soil has 

 become so dry, capillarity acts very slowly to conduct the water 

 downward into the soil. 



In the second place, most cultivated crops, in order to take 

 advantage of the general fact that summer rains do not as a rule 

 penetrate deeply into the soil, develop a system of roots ex- 

 tremely close to the surface of the ground, where momentary ad- 

 vantage may be taken of those rains which do not wet in deeply ; 

 and hence it is that in sub-humid climates, and after a dry time 

 in all climates, surface cultivation right after a rain may do posi- 

 tive injury by cutting off roots which have been developed to 

 take advantage of such rains, while at the same time the rate 

 of evaporation from the stirred soil has been increased. Here, 

 again, it is seen that rigid physical laws and conditions have set 

 limitations to the methods of tillage as a substitute for irrigation. 



MIDSUMMER AND EARLY FALL CROPS DIFFICULT TO 

 GROW WITHOUT IRRIGATION 



The fact that after early summer the surface of the ground 

 usually becomes quite dry, coupled with the other fact that water 



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