x Contents 



CHAPTER III 



THE EXTENT TO WHICH TILLAGE MAY TAKE THE PLACE 

 OF IRRIGATION (pages 117-170) 



PAGES 



The Insufficiency of Water to Irrigate all Cultivated Lands. 

 Discharge of the Mississippi river Mean annual run- 

 off for the United States Proportion of cultivated 

 fields which might be irrigated 117-120 



Most which may ~be Hoped for Tillage in the Use of Water. 

 Do soils take moisture from air to helpful extent f 

 Tillage does not diminish transpiration in plants, and 

 cannot dispense with water 120, 121 



TJie Amount of Rain Needed to Produce Maximum Crops in 

 Humid and Sub -humid Regions. Acre -inches required 

 for a pound of dry matter The amount of available 

 rainfall in the United States Effective rainfall of 13 

 states Theoretical yields which may be expected 121-125 



The Distribution of Rain in Time Unfavorable to Maximum 

 Yields. Mean yields of barley, oats and maize in 13 

 states Small mean yields, due to unfavorable dis- 

 tribution of rain 125-127* 



Methods of Tillage to Conserve Moisture often Ineffective. 

 Cultivation inapplicable Meadows and pastures Mean 

 yield of hay in 13 states Relation of yield of hay to 

 effective rainfall Tillage methods only partly appli- 

 cable to small grains 127, 128 



Tillage to Save Moisture is Chiefly Effective in Saving Winter 

 and Early Spring Rains. Late rains largely absorbed 

 by the surface three inches Roots develop close to 

 the surface in late summer 128,129 



Midsummer and Early Fall Crops Difficult to Raise without 

 Irrigation. Summer rains less effective Yields of sec- 

 ond crop clover A crop of barley and hay the same 

 season.. . 129-131 



