332 NATURE AND PROPERTIES OF SOILS 



tion may produce leaching into lower lands, an alkali condition 

 generally resulting if tlie areas so affected remain water-logged 

 for a long time. 



Very often alkali is localized in small areas called alkali 

 spots. These vary in size from a few square yards to several 

 acres. In years of good rainfall these areas may be pro- 

 ductive, but in dry years they are often quite sterile. Their 

 origin is generally due to seepage, the ground water being 

 near enough the surface to allow a concentration of salts by 

 capillarity, especially in dry seasons. 



A very peculiar type of alkali spot occurs in the Grand 

 Valley of Colorado and elsewhere, the predominant salt being 

 the nitrate, which does not usually occur in large amounts 

 as alkali. Two theories have been advanced to account for the 

 presence of the nitrate salts. One hypothesis^ is that the 

 surrounding shales are comparatively rich in nitrates and that 

 the alkali accumulation is a leaching and seepage process. The 

 other theory is biological in nature.^ Such soils are capable 

 of rapid nitrogen fixation by means of their bacterial flora. 

 The idea is advanced that the nitrogen is fixed from the air 

 very rapidly in these spots and later oxidized to the nitrate 

 form. Whatever the origin of the soluble salts the fact re- 

 mains that such spots are quite destructive, spreading very 

 rapidly until whole orchards are wiped out. 



Water used for irrigation is very often heavily charged with 

 alkali, especially where any amount of the water previously 

 applied to the soil finds its way back into the streams. At 

 Canon City, Colorado, the Arkansas River is very pure. At 

 a point 120 miles below the soluble salts have been known 



* Stewart, E., and Peterson, W., The Mtric Nitrogen Co%te%i of the 

 Country Boclc; Utah Agr. Exp. Sta., Bui. 134, 1914. 



Also, Fwrther Studies of the Nxtno Nitrogen Content of the Country 

 BocTc; TJtsih Agr. Exp. Sta., Bui. 150, 1917. 



^Headden, W. P., The Fixation of Nitrogen in Colorado Soils; Colo. 

 Agr, Exp. Sta., BuL 186, 1913, 



Saekett, W. G., and Isham, E. M,, Origin of tJie Niter Spots m 

 Certain Western Soils; Science, N. S., Vol. 42, pp. 452-453, 1915. 



