182 Irrigation and Drainage 



tries where there is an abundant and timely distribution 

 of rain, or where irrigation is practiced, the number 

 of plants per acre may be so far increased that the 

 limiting factors become the available plant-food stored 

 in the soil, the amount of sunshine which falls upon 

 the area, or the circulation of air about the assimilat- 

 ing foliage. 



It is very evident that were the amount of available 

 water for crop production the only factor which de- 

 termines the number of plants which can be grown per 

 unit area, the methods of irrigation would make it pos- 

 sible to greatly increase the yield of almost any crop 

 in the most humid of climates. But there are many 

 limiting factors which set rigid bounds beyond which 

 irrigation may not pass. 



Sufficient breathing room in the soil. Since the roots 

 of all cultivated plants demand free oxygen in the soil 

 for their respiration, and since not only the possible 

 quantity of free oxygen in the soil, but the rate at 

 which it may be supplied, decreases as the quantity of 

 water in the soil increases, and since the closer the 

 plants are set upon the ground the more densely crowded 

 must the roots be in the soil, and the more rapid must 

 be the interchange of gases between the soil and the 

 air above in order to meet the increased demands for 

 growth, it is plain that the demand for free oxygen in 

 the soil sets a rigid limit beyond which closer planting 

 must not be pushed. 



It must be kept ever in mind that the soil is like a 

 very poorly ventilated assembly hall, which may easily 

 be so crowded as not only to produce discomfiture to 



