Arizona Agricultural Experiment Station. 182 



Now, since 20,000 pounds per acre of soluble salts in the sur- 

 face four feet, — more or less according to soil, drainage, the na- 

 ture of the salts and kind of crop grown, is about the average 

 limit for ordinary crops, it follows that if this soluble matter 

 •should all remain in the soil its effects would be soon felt. 



Fortunately the districts watered with seepage are narrow in 

 proportion to their length and have excellent natural drainage. 

 Moreover, the occasional flood waters which come down from 

 above will undoubtedly in future prove a means of flooding alkali 

 lands and leaching out their excessive salts. 



In the table these waters are all observed to contain "per- 

 manent hardness," which consists in part of the sulphate of lime. 

 This compound, in moist, well-aerated soil, is an antidote for 

 black alkali, which is found in some parts of the irrigated dis- 

 tricts. The application of these waters to irrigated lands may 

 therefore, in some cases, actually improve them so far as this most 

 injurious form of alkali is concerned. 



The silt is of interest as affecting the fertility of irrigated 

 lands and as being, also, a possible menace to the life of water 

 storage reservoirs. The fertilizing value of irrigation silts the 

 world over is well known; although excessive amounts are an 

 inconvenience in irrigation. 



The average amount of silts in the Salt river supply from 

 Aug. 1, 1899, to Aug. 4, 1900, by weight and by volume, was, 

 by weight ,iJ of the water; by volume .3 . This amount of 

 silt is unquestionably less than the average on account of the un- 

 usually low water prevailing during most of the time of sampling. 

 The Salt river, however, is undoubtedly far less silty than the 

 Gila, and this element of doubt in connection with the life of res- 

 ervoirs correspondingly less. 



The figures for total nitrogen and nitrogen as nitrates are 

 also of special interest, our soils being usually deficient in nitro- 

 gen. The average total nitrogen in the Salt river supply for one 

 year was found to be 3.25 parts in 1,000,000 of water, including 

 that which was contained in the silt. Of this amount 1.04 parts 

 per million existed in the form of nitrates. Differently stated, 

 this means that, in the course of one year, four acre feet of average 

 quality water applied to land in Salt River valley would contain 35.39 



