548 



Journal of Agricultural Research 



Vol. X. No. II 



Calcium nitrate was added to 20 per cent solutions of sodium nitrate 

 in amounts shown in Table V, and these solutions were shaken up with an 

 excess of calcium carbonate until they had reached equilibrium. The 

 sodium carbonate was then determined. 



Calcium chlorid was also added to 10 per cent solutions of sodium 

 chlorid, and calcium sulphate to 20 per cent solutions of sodium sulphate 



/S(> 



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 A/az 



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sc'^cryo/VS' 



5'Q. 



in the same way. The 

 curve with calcium 

 chlorid follows very 

 closely the curve with 

 sodium nitrate. Elim- 

 inating for the present 

 other modifying factors, 

 it will be seen that in 

 the presence of 0.5 per 

 cent of either of the sol- 

 uble lime salts, calcium 

 chlorid or calcium sul- 

 phate, the quantity of 

 sodium carbonate 

 formed from the reac- 

 tion of calcium carbon- 

 ate with sodium chlorid 

 or sodium nitrate, is 

 very slight. In other 

 words, if such reactions 

 are responsible for the 

 formation of normal 

 sodium carbonate or 

 black alkali in the soil, 

 and it will be shown 

 later that such reactions 

 unquestionably do take 

 place, even in the pres- 

 ence of the carbon di- 

 oxid of the soil, it is an 

 essential condition that 

 the calcium-chlorid content of the soil solution be reduced below 0.2 per 

 cent and the calcium nitrate content below 0.5 per cent. The removal of 

 the calcium chlorid and the calcium nitrate under natural conditions might 

 result either from drainage or from surface evaporation. In the latter case 

 the salts would be brought to the surface in solution and would accumulate 

 as a crust or efflorescence. This is exactly what happens in nature. The 

 calcium chlorid , or " slick spots, ' ' which are so often mistaken for black alkali 

 and which are so common in the West, are no doubt formed in this wav. 



./ ^ .^ .<? .^ G .7 



Fig. 2.— Graphsshowingthedepressionof sodium carbonate formed by 

 the reaction of calcium carbonate with sodiimi chlorid, sodium nitrate, 

 or sodium sulphate when a soluble calcitmi salt containing a common 

 anion is added. 



