ALKALI SOILS. 



423 



sist. Where the rainfall is abundant, that portion of the salts 

 corresponding to " sea salts " is leached out into the bottom 

 water, and with this passes through springs and rivulets into 

 the country drainage, to be finally carried to the ocean. 1 An- 

 other portion of the salts formed by weathering, however, is 

 partially or wholly retained by the soil; it is that portion chiefly 

 useful as plant food. 



It follows that when, in consequence of insufficient rainfall, 

 all or most of the salts are retained in the soil, they will contain 

 not only the ingredients of sea-water, but also those useful to 

 plants. In rainy climates a large portion even of the latter 

 is leached out and carried away. In extremely arid climates, 

 on the contrary, the entire mass of the salts remains in the 

 soils; and, being largely soluble in water, evaporation during 

 the dry season brings them to the surface, where they may 

 accumulate to such an extent as to render ordinary useful 

 vegetation impossible; as is seen in "alkali spots," and some- 

 times in extensive tracts of ' alkali desert.'' Three compounds, 

 viz. the sulfate. chlorid and carbonate of sodium, usually form 

 the main mass of these saline efflorescences. Magnesium sul- 

 fate (Epsom salt) is in many cases a very abundant ingredient; 

 some calcium sulfate is nearly always present, and calcium 

 chlorid is not infrequently found. 



In some cases the above salts are in part at least derived from the 

 leaching of adjacent or subjacent geological deposits impregnated with 

 them at the time of their formation. Such is the case in portions of 

 Wyoming, Colorado and New Mexico, in the Colorado river delta, and 

 in the Hungarian IMain ; and it is in these cases especially that the 

 chlorids of calcium and magnesium also form part of the saline mixture. 



Geographical Distribution of Alkali Lands. In looking over 

 a rainfall map of the globe' 2 we see that a very considerable 

 portion of the earth's surface, forming two belts to poleward 

 of the two tropics, has deficient rainfall; the latter term being 

 commonly meant to imply any animal average loss than jn 

 inches (500 millimeters). The arid region thus defined in- 

 cludes, in North America, most of the country lying west of 

 the one hundredth meridian up to the Cascade Mountains, and 



1 See Chapter 2, p. 26. J See above, chapter 16, p. 294. 



