ALKALI SOILS 331 



interfere with the growth of most crops might completely pre- 

 vent the development of useful plants if the alkali were black. 



175. Origin of alkali. — While the presence of alkali and 

 its influence on plants has been known for centuries, it is 

 only within recent years that its probable mode of origin has 

 been understood. The soluble salts have undoubtedly come 

 from the materials which have formed the soils, the reactions 

 being as complex as the ordinary transformations which take 

 place in soil formation. 



Some soils have been laid down as deltas in arms of the 

 ocean. If these bodies of water later are cut off from the sea 

 and gradually dry up under arid conditions, an alkali soil 

 will be left. In a similar way saline lakes may disappear and 

 soils heavily charged with alkali will result. 



The commonest mode of origin for alkali soil is through 

 ordinary weathering under conditions of aridity. Almost any 

 rock will give rise to soils rich in alkali salts if leaching is not 

 a feature in the weathering processes. In western United 

 States the origin of much of the soil affected to the greatest 

 degree with alkali is associated with strata originally carrying 

 much soluble material. When such rock forms soil, the alkali 

 arises not only from the decomposition of the minerals of 

 which the rock is composed, but is greatly reinforced by the 

 soluble salts already present. The Cretaceous and Tertiary 

 beds in Utah, Colorado, and Wyoming are of this character, 

 having been laid down in brackish water. They naturally 

 give rise to soils high in alkali. 1 



One fact that is often overlooked in practice is that the 

 amount of alkali in the surface layers of soil may be greatly in- 

 creased by improper handling. Rapid evaporation after rain 

 or irrigation will carry the soluble salts toward the surface and 

 deposit them near to or in the root zone. Again, over-irriga- 



1 Stewart, E., and Peterson, W., Origin of Alkali; Jour. Agr. Res., 

 Vol. X, No. 7, pp. 331-353, 1917. See also, Breazeale, J. F., Forma- 

 tion of Black Alkali in Calcareous Soils; Jour. Agr. Res., Vol. X, No. 

 11, pp. 541-589, 1917. 



