588 Journal of Agricultural Research voi. x, no. n 



the two salts then took place (NajSO^ + CaCOgti^NajCOg + CaSO^), and the 

 products of the reaction were brought to the surface by capillarity. In 

 the case of No. 2, the sodium sulphate, coming up through the sand by 

 capillarity, had to pass through the thin layer of calcium carbonate. 

 At this point the same reaction took place, and the sodium carbonate 

 was brought to the surface. 



This experiment illustrates what often takes place in the field and is a 

 possible explanation of why a lime hardpan almost invariably accompa- 

 nies black alkali. The reverse of this, however, is not true. Black 

 alkali does not always accompany hardpan, for the obvious reason that 

 sodium chlorid or sodium sulphate are not always present in sufficient 

 amounts in the soil or in the irrigation water to bring about the reaction. 



CONCLUSIONS 



(i) In the reaction between sodium nitrate (or sodium chlorid or 

 sodium sulphate) and calcium carbonate, resulting in the formation of 

 sodium carbonate, the presence of relatively small amounts of calcium 

 nitrate or calcium chlorid in the reaction impedes and may prevent the 

 formation of sodium carbonate. 



(2) The presence of a saturated solution of calcium sulphate in this 

 reaction does not entirely stop the formation of sodium carbonate. 



(3) Sodium nitrate, sodium chlorid, and sodium sulphate in the 

 presence of carbon dioxid react with calcium carbonate with the foi-ma- 

 tion of sodium bicarbonate. 



(4) The presence of relatively small amounts of calcium nitrate or 

 calcium chlorid in this reaction impedes and finally prevents the forma- 

 tion of sodium bicarbonate. 



(5) The presence of calcium sulphate has no effect in preventing the 

 formation of sodium bicarbonate when sodium sulphate or a mixture 

 containing sodium sulphate reacts with calcium carbonate. 



(6) A field application of gypsum will probably have no effect in over- 

 coming black alkali if the soil already contains soluble sulphates in 

 appreciable amounts, or the irrigation water contains these salts. 



(7) Sodium nitrate, sodium chlorid, and sodium sulphate increase the 

 solubility of calcium carbonate in the soil. 



(8) Sodium nitrate, sodium chlorid, and sodium sulphate react with 

 calcium carbonate in the soil with the formation of sodium carbonate 

 ("black alkali"). 



(9) Sodium carbonate, formed by the above reaction, decomposes the 

 organic matter of the soil. 



(10) Calcium carbonate has a slightly destructive action upon the 

 organic matter of the soil. 



(11) Sodium carbonate is much more destructive upon organic matter 

 than sodium bicarbonate. 



