108 



AERATION AND AIR-CONTENT. 



Table 27. 



The inverse relation of water and soil-air is chiefly discussed in 

 the following sections, but the significance of irrigation-water in this 

 connection has been largely determined by Howard and Howard in 

 their investigations of aeration as a primary factor in agriculture. 



Widtsoe (1914 : 249) has probably been the first to show that there 

 is a steady decrease in the yield of wheat per inch of water as the 

 irrigation of a field is increased, and that excessive irrigation may 

 produce an actual decrease in the total yield, though he did not rec- 

 ognize that this was chiefly due to faulty aeration. The rapid de- 

 crease in production for each inch of water used is shown in table 27. 



Two irrigations amounting to 

 7.5 inches are regarded as sufficient 

 for a crop of wheat on deep soil, 

 and 4 to 5 irrigations, totahng 18 

 inches, on shallow gravelly soil. 

 On many soils a single irrigation 

 of 5 inches is better. 



Howard and Howard (1915, 

 1919) have pubHshed two bulletins 

 on the saving of irrigation-water 

 in wheat-growing, which deal with 

 the principles underlying water- 

 saving, and with experiments at 

 Quetta and in India. The six principles are the following: 



(1) The irrigation- water available should be spread over the 

 largest possible area. 



(2) Irrigation-water must be applied in such a manner as to inter- 

 fere as little as possible with the natural aeration of the soil. 



(3) Heavy waterings reduce the proportion of grain to total crop. 



(4) The growth-period of wheat is increased by heavy watering. 



(5) When the water-supply is limited, the root development of 

 the wheat crop must be deep. 



(6) The soil-moisture must be preserved as far as possible by a 

 surface mulch of dry soil. 



While all of these relate to aeration as well as water economy, the 

 first three have to do directly with a proper supply of soil-air. Other 

 things being equal, the soil-air is increased as the irrigation-water 

 is diminished, and with respect to the plant alone, the best irrigation 

 method involves the most effective compromise between too much 

 water and too little air (table 28). 



The economic waste involved in using irrigation-water beyond 

 the optimum is threefold. The most serious waste occurs when an 

 actual reduction of the yield per acre takes place, but scarcely less 

 important is the waste resulting from a rapidly diminishing return 

 per acre-inch of water used. In average seasons such waste amounts 

 at least to much of the cost of the water, and may amount to the 



