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water is needed for these nitrif \ ing organisms, but, on the other hand, 

 too much water stagnates and destroys them. Thus it is that there is 

 a very delicate adjustment to be preserved in respect to the amount of 

 moisture in order to procure the best results. These conditions the 

 successful irrigator learns by experiment and failure, and unconsciously 

 follows certain rules which he is usually unable to put into words. 



There has been very little progress in the practice of irrigation from 

 the methods of ancient times. This is due largely to the fact that the 

 men who are now bringing new lands under ditch have for the most part 

 received their training as farmers in humid regions, and find it difficult 

 to unlearn many of the facts which they regard as fundamental, and 

 to reverse the habits of half a lifetime. They hesitate to adopt the 

 methods of the Indians and Mexicans, despising these as crude or 

 childish. Nevertheless these primitive peoples have, through the expe- 

 rience of generations, acquired certain ways which are worthy of study, 

 particularly in the direction of using the smallest possible amount of 

 water in oases on the desert. When they have plenty of water, the 

 Mexicans use it wastef ully ; but where the amount is extremely limited, 

 some of them, particularly the agricultural Indians of the South-west, 

 have acquired the art of utilising every drop. Even the drippings 

 from the family water jar are arranged to fall upon a growing plant, 

 and the moist spots are carefully guarded for the growing of corn or 

 beans. 



The amount of water required for raising crops varies according to 

 soil and other conditions. The plant itself needs a certain minimum 

 supply in order to receive and assimilate its food and to keep up trans- 

 piration. A far larger quantity is required to saturate the surround- 

 ing soil to such a degree that the vitalising processes can continue. 

 The soil is constantly losing water by evaporation and by seepage, so 

 that the amount which the plant takes from it is relatively smaUJ 

 Nevertheless, the moisture must be maintained within narrow limits 

 in order to produce the most favourable conditions of plant growth. 



Experiments have been made to determine exactly how much water is 

 needed in order to keep the soil in proper condition for plants of different 

 character. Among the most important investigations are those by 

 Professor F. H. King, who has found by direct measurement that from 

 300 to 500 pounds of water are required for each pound of dry mat- 

 ter produced ; in other words, for each ton of hay raised upon an acre, 

 300 to 500 tons of water must be furnished either by rainfall or by 

 artificial means. 



Water covering an acre 1 inch in depth weighs about 113 tons, and 

 to produce one ton of hay the depth of water required is approximately 

 from 3 to 5 inches. It is necessary to furnish at least this amount, 

 and sometimes several times as much, in order to produce a crop. 

 The actual amount used in producing 5 tons of barley hay to the acre 

 has been about 20 inches in depth. Much depends upon the permea- 

 bility of the soil, and its ability to hold water. 



The quantity of water used in irrigation is usually stated in one of 

 two ways— either (1) in terms of depth of water on the surface, or 

 (2) in quantities of flowing water through the irrigating season. The 

 first method is preferable, since it is susceptible of mora definite con- 

 sideration, and is also more convenient for comparison with figures for 



