360 THE REGOLITH 



of limestone or carbonate of lime, and that an exchange has 

 occurred in which either common salt or Glauber salt have trans- 

 ferred their acidic components to lime and have become car- 

 bonates instead. . . . Yet the simple explanation of the con- 

 trary reaction was given and published as early as 1826 by 

 Schweigger. In 1859 it was again observed by Alex Muller, 

 in a different form, but neither of these chemists, nor any of 

 their readers, appear to have perceived the important bearing of 

 this reaction, not only upon the formation of the natural depos- 

 its of carbonate of soda, but also upon a multitude of processes 

 in chemical geology. Without going into details ... it may 

 be broadly stated that the formation of carbonated alkalies oc- 

 curs whenever the neutral alkaline salts (chlorides or sulphates) 

 are placed in presence of lime or magnesia carbonates and car- 

 bonic acid, or of alkali 'supercarbonates' (hydrocarbonates) con- 

 taining even a slight excess of carbonic acid above the normal 

 carbonates, the latter being the actual condition of all natural 

 sodas. ' ' ^ 



"We have thus far considered only those elements of the soil 

 that are derived directly from the rocks from which they are 

 formed. 



To this list we should add the element nitrogen, not so much 

 on account of its quantity, as its value as plant food and of the 

 great economic value of some of its compounds. The common 

 forms under which this element exists, are (1) atmospheric 

 nitrogen, a colorless, tasteless, and innocuous gas which forms 

 some three-fourths by bulk of the air we breathe, and (2) the 

 nitrogen of the soil, where it exists in at least three distinct 

 forms, (1) organic nitrogen, (2) as ammonia or ammonia salts, 

 and (3) as nitric acid. 



The average amount of nitrogen present in agriculture soils 

 is given by authorities as varying from 0.1% to 0.3%, though 

 occasionally, as in certain soils rich in organic matter, 4 or 5%. 

 Of these forms only the ammonia salts and nitric acid are of 

 direct value for plant food. Nitrogen, in the form of nitrate 

 of soda, forms an important mineral fertilizer, as noted on p. 67* 



The extraordinary richness in nitrates of the soils in tropical 

 countries, and particularly in South America, has often been 



=^See furtlier the Mineral Constituents of tlie Soil Solution, by F. K. 

 Cameron and J. M. Bell, Bull. 30, Bureau of Soils, IT S, Dept. of Agricul- 

 ture, 1905. 



