IRRIGATION 263 



with the value of the dissolved materials, one of the great advantages 

 of irrigation is made evident. By this addition of "plant food from 

 year to year cropping may continue indefinitely without depleting 

 the soil. Some streams are exceptions to this rule, however. 



Time of Irrigation. The irrigation of crops may take place at 

 various times, depending upon the crop grown and the object to 

 be accomplished. Theoretically the soil should be supplied with 

 just sufficient water to maintain optimum conditions for growth 

 and maturity. This is a condition to he desired, whether ever at- 

 tained or not; however, this is rarely possible, since the supply of 

 water frequently runs so low that during part of tbe growing 

 season it is not adequate for the purpose. 



Irrigation may be done either when the crop is not growing, in 

 the fall, winter or early spring, or when the crop is growing during 

 the summer. In the former case the ohject is to obtain the water 

 when the demand for it is not so great and store it in the soil for 

 use the next season. It may he done immediately after harvest and 

 from then till spring. Winter irrigation is not advisable when the 

 soil is frozen, as much of the water may he lost, but where (he win- 

 ters are mild it may be practiced to good advantage. 



Alfalfa and wheat should not he flooded during the winter in 

 cold climates. 



Irrigation water may be applied early in the spring to save some 

 of the water of the spring floods caused by the melting snows of 

 the mountains. This would be largely lost unless reservoirs have 

 been built to store it for summer use. The time and frequency of 

 irrigation depend upon the crop. In Arizona orchards receiving 

 fall and winter irrigations have produced well without any further 

 application of water. Alfalfa should be irrigated several times, a 

 few days before cutting and again soon after the crop has been 

 harvested. Wheat and other small grains, beans and peas if planted 

 in a soil well filled with moisture need little or no irrigation till 

 flowering time. This permits a good root system to develop. Early 

 irrigation lessens the proportion of grain to straw. 



Amount of Water to Apply. As a general rule the more 

 water that is applied to a soil, within practical limits, the larger 

 amount, of dry matter it produces. The problem is not to go beyond 

 the point of most profitable returns. Tin's point has not yet been 

 determined. Tt is very difficult of determination, since it varies 

 with the crop, the soil, rainfall and other conditions. 



Usually more water is applied than is necessary and certainly 



