Sept. lo, 1917 Formation of "Black Alkali " 543 



edge of the moisture plane, or where the moist soil of the surface shades 

 into the dry subsoil. While it is quite likely that capillarity had httle 

 or nothing to do in the formation of this hardpan, which occurred under 

 conditions of limited rainfall, it is, however, usually within easy reach 

 from the surface of the capillary pull which would occur in a saturated 

 soil, especially under conditions of irrigation. The importance of this 

 calcareous hardpan in the study of the formation of black alkali will be 

 brought up later. 



Of all the alkali elements existing in this area, sodium, occurring as 

 sodium chlorid, sodium sulphate, sodium carbonate, or sodium bicar- 

 bonate, and sodium nitrate, is the most common. This is due to the 

 fact that soils originated largely by the evaporation of marine lagoons 

 or landlocked seas, or were derived primarily from the so-called granites 

 or more properly, diorite (soda-lime feldspar and hornblende). In 

 these same soils, except in cases where a too high salt content prohibits 

 activity, nitrification is usually rather intense. The presence and 

 accumulation of nitrates, from this as well as from other sources, is 

 usually noticeable in such cases. 



Sodium carbonate (black alkali) is frequently met with in this area. 

 As this is the most toxic salt that exists in appreciable amounts in alkali 

 soils, its origin has created a good deal of speculation. It seems prob- 

 able that all cases of black alkali are not due to the processes of rock 

 disintegration. While the toxic action of sodium carbonate upon 

 plants is much greater than that of sodium bicarbonate, they will not 

 be differentiated in this discussion. It must be remembered that 

 sodium bicarbonate is formed from sodium carbonate by the addition 

 of carbon dioxid. Any cause which will liberate the carbon dioxid, 

 such as the precipitation of the salt upon the surface by the evapora- 

 tion of the water in which it is dissolved, will tend to convert the bicar- 

 bonate to the carbonate. The natural tendency of the bicarbonate is 

 to become a carbonate, and, as carbon dioxid exists in all soils in 

 appreciable amounts, it seems probable that the primary step in the 

 formation of black alkali is often the formation of sodium carbonate. 



REACTION BETWEEN SODIUM SALTS AND CALCIUM CARBONATE 



It is well known that sodium salts will react with calcium carbonate, 

 with the formation of sodium carbonate or bicarbonate and the corres- 

 ponding calcium salt. The extent of this reaction and its bearing 

 upon the formation of black alkali in nature has been taken up in this 

 investigation. 



Cameron (i) has described the probable formation of different types of 

 alkali, and later Seidell (2) , working under his direction, studied the reac- 

 tion between sodium chlorid, sodium sulphate, and calcium carbonate. 

 They drew air through the solutions until they had reached equilibrium 

 and determined the amounts of sodium bicarbonate thus formed. 



