OBSERVED TOXIC LIMITS 229 



Hilgard (17) (19) states that although 685 parts per mil- 

 lion (40 grains per gallon) of the common alkali salts should 

 be the limit under most conditions, the nature of the 

 salts will modify the limits considerably. As little as 342 

 parts per million of sodium carbonate has in some instances 

 caused serious injury in three or four years, while as much 

 as 2739 parts per million of the less toxic salts would not 

 be harmful. From his work in California, Mackie (24) 

 states that where the salts are principally bicarbonate 

 and chloride of sodium, irrigation water containing more 

 than 600 to 700 parts per million of salt should not be 

 applied except to porous, well-drained soils. Guthrie (13) 

 considers 500 parts per million of sodium carbonate as a 

 tolerable quantity of this salt even when as much as 150 

 parts per million of sodium chloride are also present. 



Where the salts are more of the sodium-sulphate type, 

 larger quantities are permissible. Forbes (18) states that 

 with good drainage 1000 parts per million of salts in ir- 

 rigation water is an objectionable but permissible degree 

 of salinity for the soils of the Salt River, Arizona. In 

 the Pecos Valley (26) 2500 parts per million to 3000 parts 

 per million of salts were considered the danger zone where 

 about 50 per cent of the salts in the water were of sodium 

 mostly sodium chloride and sodium sulphate. Good 

 drainage in the upper part of the valley makes possible 

 the use of water of higher salinity than is possible in lower 

 parts of valleys where the soil is heavier and likely to 

 contain more alkali. Land, after being irrigated five 

 years with water containing 3900 parts per million of salts, 

 was abandoned because of the accumulation of alkali and 

 seepage water. 



Experiments in Wyoming (31) show that where only 

 small quantities of water are added, practically all of the 



