183 Eleventh Annual Report. 



pounds of nitrogen, mostly in the silt, but 11.32 pounds being dis- 

 solved as nitrates in the water. If 40 per cent of this latter por- 

 tion were lost in the seepage water, there would be left a net 

 addition of 30.87 pounds of nitrogen to an acre of sufficiently irri- 

 gated land in one year. 



Now, a crop of four tons of barley hay requires about 117 

 pounds of nitrogen, 50 bushels of wheat require not far from 59 

 pounds, and a crop of 10 tons of sugar beets about 57 pounds. 

 It is thus evident that the irrigating water of this representative 

 region contributes materially to the nitrogen requirements of ty- 

 pical crops. This fact, doubtless, accounts in large measure for 

 the fertility of irrigated desert soils, even when first put under 

 cultivation. 



ARTESIAN WATERS. 



The artesian water supply which has been so vigorously de- 

 veloped on the eastern slope of Graham mountain above Safford, 

 Thatcher, and Pima, and also the artesian wells in the San Pedro 

 valley south of Benson, have bean under observation during the 

 year. 



The examinations of these waters with reference to their use- 

 fulness for irrigating purposes reveal certain important facts re- 

 garding them. In the table of analyses it will be observed that, 

 although the total soluble salts are, with exception, not excessive, 

 carbonate of soda, or "black alkali," is uniformly present. The 

 Mud Spring well, for instance, with 19.6 parts per 100,000 of 

 sodium carbonate, contains 533.6 pounds of this ingredient in one 

 acre foot of water. Calculating on three acre feet of water a year, 

 this would mean a total of 1600.8 pounds added, which in the up- 

 per three feet of soil would amount to. 0134% of the soil. But . i%of 

 black alkali is the maximum amount allowable in the soil in or- 

 dinary farming; therefore, if all the black alkali in this water re- 

 mained in the top three feet, its effect would be severely felt in 

 about eight years. If sensitive crops were grown, the time would 

 tend to be less; with good drainage, bad effects would be delayed. 

 Fortunately the waters of this district as a rule contain much less 

 of sodium carbonate, but in any case its presence in an irrigating 

 water is a matter for watchfulness. 



