DEAIFAGE OF lEEIGATED SHALE LAND. 



Table II. — Analyses of seepage toaters through virgin shales. 

 (No land lying above ever under cultivation.) 



Substance. 



Milligrams per liter (parts per 

 1,000,000). 



1915, 

 Nov. 18. 



1915, 

 Nov. 12. 



1916, 

 Apr. 29. 



Sulphuric acid (SO4) 



Carbonic acid (CO3) 



Bicarbonic acid (HCO3) . 



Nitric acid (NO3) 



Chlorine(Cl) 



Calcium (Ca) 



Magnesium (Mg) 



Sodium (Na) 



4,236 



434 

 4,004 



480 

 1,110 



523 

 1,725 



21,800 



577 

 1,038 



567 

 2,481 

 6,376 



37,612 



77 

 710 

 1,687 

 2,783 

 535 

 2,556 

 15,318 



Total. 



12,512 



33, 784 



61,278 



Hypothetical combinations. 



Sodium carbonate (Na2C03) 



Magnesium chloride (MgCl) 



Magnesium sulphate (MgS04) 



Sodium nitrate ( NaN O3) 



Sodium chloride (NaCl) , 



Sodium sulphate (Na2S04) 



Calcium sulphate (CaS04) 



Calcium bicarbonate (Ca(HC03)2) . 

 Calcium carbonate (CaCOs) 



170 



136 



148 

 2,402 

 5,489 



609 



3,287 



577 



12,282 



791 



1,711 



16,723 



979 



1,128 



12, 653 

 2,313 

 4,588 



39,620 



1,025 



943 



Total. 



12,512 



33,784 



61, 278 



These are 'all bad waters, as only three of the 13 samples show less 

 than 10,000 parts per million, or 1 per cent, of the soluble salts, and 

 one of these is but slightly below this figure. The average for the 13 

 samples is slightly in excess of 23,700 parts per million, or 2.37 per 

 cent. 



An acre-foot of water weighs 2,722,500 pounds and with a salt 

 content of 23,700 parts per million will contain 64,523 pounds of 

 salt. Assuming the weight of an acre-foot of soil at 4,000,000 pounds, 

 the salt that would be left in the soil by evaporation from it of but a 

 single acre-foot of this water would be equivalent by weight to 1.6 per 

 cent for the top foot of the soil. If this quantity were distributed 

 throughout a depth of 4 feet, it would mean an average of four-tenths 

 of 1 per cent. It is conservative to assume an annual evaporation of 

 36 inches under such climatic conditions as exist in those sections of 

 the United States to which this bulletin refers, so that it would re- 

 quire but one season to deposit enough salts, by evaporating the aver- 

 age of the waters represented by Tables I and II, to make the situa- 

 tion three times as bad as the above. Allow surface evaporation, re- 

 sulting from poor drainage, to continue for a number of years and 

 the situation with respect to alkali is rendered extremely serious. 



Many investigators, working independently, have attempted to 

 establish a comparative standard relative to the tolerance of plants 

 for the different alkali salts present in a soil. Owing to the variety 



70250°— Bull. 502—17 2 



