General Discussion 223 



Irrigating sediments alone, therefore, considered in their 

 general relation to amounts of copper which cannot be prevented 

 from reaching irrigated fields, are sufficient in quantity to re- 

 duce ultimately the amounts of copper observed below 0.01 per 

 cent in the soils of this district. Since 0.01 per cent is a safe 

 minimum, river sediments, alone, incorporated with the soil are 

 probably sufficient to ameliorate gradually existing accumula- 

 tions of copper salts and to take care of further contributions 

 in soluble form which cannot at present be avoided. 



Effect op Cultivation upon Alfalfa 



Finally, it is of interest to observe the improvement in a 

 field of alfalfa, in the district studied, between the years 1005 

 and 1916. 



June 23, 1905, the writer carefully measured, cut and weighed 

 a representative plot of alfalfa in William Gillespie's field near 

 Solomonville, Arizona. This field was suffering from an accu- 

 mulation of tailings, the depreciation in yield at the upper ends 

 of alfalfa lands being conspicuously evident. Following the 

 exclusion of tailings from the irrigating supply in 1908, and with 

 a cultivation each winter with a disk or a spring-tooth harrow, 

 the condition of the field gradually improved until, June 13, 1916, 

 the writer returned and again measured, cut, and weighed the 

 identical plot of alfalfa that had shown bad effects eleven years 

 before. Following are the data, with diagrams, relating to these 

 two cuttings of alfalfa, which are representative for the district 

 within which tailings were deposited. 



1. Alfalfa seriously affected by tailings, June 23, 1905. 



Three lands in William Gillespie's field east of house, near 

 Solomonville, under Montezuma Ditch, out of Gila River. A good 

 stand of alfalfa five years old. Heavy adobe soil ; field never 

 disked. 



The three lands observed were, over all, 95 feet wide, and 

 divided into plots 100 feet long from top to bottom of field. Ten 

 feet next the ditch was discarded because of banks and bare spots, 

 and the extreme lower portion of the field because of roadways. 

 A portion of plots 6 and 7 was discarded on account of Johnson 

 grass. 



