78 SOILS. 



("kara") is given it in Arabia and Asia Minor, whence im- 

 pure soda has long been imported into Europe ; while in north 

 India it forms part of the " reh " salts that incrust large areas 

 (usar lands) in the Indo-Gangetic plain. 



The natural mineral always contains an excess of carbonic acid over 

 the " normal " salt, nearly in the proportion of four parts of carbonic 

 dioxid to three of soda ; it is sometimes designated as sesqui-carbonate. 

 In hot sunshine it may lose most of this excess for a time ; while within 

 the soil itself it may, in presence of abundant carbonic acid, become 

 temporarily converted wholly into hydrocarbonate or " bicarbonate," 

 which is less corrosive than the monocarbonate or common salsoda. 



Injury caused in soils. Like common and Glauber's salt, 

 carbonate of soda is always an unwelcome soil ingredient ; 

 more so, in fact, than either of the other two, since less than a 

 tenth of one per cent is sufficient to render certain soils 

 wholly untillable, by the deflocculation or puddling of the clay; 

 at the same time rendering it impervious to water. It is by 

 far the most injurious ingredient that ordinarily occurs in 

 otherwise good, arable soils; for in addition to the physical 

 effect just mentioned, it dissolves the humus-substance of the 

 soil, forming an inky-black solution which, especially when 

 evaporating on the surface and forming black spots, has given 

 rise to the popular name of " black alkali." As will be more 

 fully explained hereafter, wherever such is the case, the first 

 step necessary toward reclamation is the transformation of the 

 carbonate of soda, at least in part, into the relatively innocu- 

 ous sulfate, by means of gypsum in. the presence of water; 

 while carbonate of lime remains in the soil. 



In its direct action on the plants themselves, soda is also 

 most injurious; as when accumulated to any extent near the 

 surface by evaporation it will corrode the root-crown or stem, 

 and sometimes completely girdle the same, destroying the 

 bark. Farther details on this subject are given in chap- 

 ter 22. 



Epsomitc, or Epsom salt, or sulfate of magnesia, is another 

 one of the water-soluble minerals frequently found efflorescent 

 on the surface of the ground ; more commonly in saline sea- 

 shore lands than in the alkali region proper, although it is 



