III 



always, soils containing alkali also contain accumulations of the 

 less soluble salts, calcium carbonate, or calcium sulphate, or a 

 mixture of the two. These substances, sometimes cementing 

 the less soluble mineral components of the soil, sometimes almost 

 pure, are found in layers more or less continuous, and from a 

 fraction of an inch to several feet in thickness, in a position 

 approximately parallel to and at a moderate depth below the 

 surface of the soil. In such cases these layers form a "hard-pan" 

 and frequently the treatment of this type of hard-pan is the most 

 difficult and vexing problem in the management of alkali-bear- 

 ing soils. 



The origin of alkali is often uncertain. In some cases the 

 geological evidences in the area make it certain that the alkali 

 came from the desiccation of former bodies of sea water which 

 had become isolated from the ocean. In other cases the alkali 

 appears to come from the desiccation of lakes which are the de- 

 positories of the drainage of a surrounding area, and which have 

 no outlet to the sea. In still other cases it has been supposed 

 that the alkali is derived from wind-borne sea-spray. Various 

 explanations of a more or less special character with regard to 

 particular localities or circumstances are to be found in the litera- 

 ture. 1 



The chemical principles involved in the desiccation of a body 

 of sea water are now pretty well understood, owing mainly to 

 the investigations of van't Hoff, Meyerhoffer, and their co- 

 workers. 2 The salts in sea water and those constituting "white 

 alkali" are mainly the chlorides and sulphates of sodium, potas- 



1 An interesting case is the Billings Area, Montana, where the alkali 

 seems to be derived from the oxidation, solution and subsequent hydroly- 

 sis of the pyrites and marcasite of the neighboring Pierre shales. The 

 sulphuric acid thus formed, leaching through shales and sandstones, takes 

 up various bases and the predominating salts in the alkali of this area 

 are the sulphates of sodium and magnesium. 



2 Zur Bildung der ozeanischen Salzablagerungen, von J. H. van't Hoff, 

 Braunschweig, 1905-09. For a detailed discussion of these results with 

 reference to alkali deposits see : Calcium sulphate in aqueous solutions, 

 by Frank K. Cameron and James M. Bell, Bull. No. 33, Bureau of Soils, 

 U. S. Dept. Agriculture, 1906. 



