ALKALI SOILS, 451 



the solid state. The calcium chlorid not uncommonly found in 

 some alkali regions is undoubtedly the product of the above re- 

 action. 



As the saline solutions in the soil are mostly quite dilute, and 

 calcic carbonate is always present, it follows that whenever 

 under the influences which favor the oxidation of organic mat- 

 ter in the soil, and the activity of the plant roots, carbonic gas 

 is formed somewhat copiously, alkali sulfates and chlorids 

 present may be partially or wholly transformed into carbonates 

 within the soil. As a matter of fact, it is found that this trans- 

 formation occurs most readily in the moister portions of the 

 soil and subsoil, and invariably so when an alkali soil is 

 " swaniped " by excessive irrigation or rise of bottom water; 

 while the reaction is again reversed whenever free access of air 

 reduces the carbonic dioxid below a certain point. It thus be- 

 comes intelligible why in the diagrams showing the distribu- 

 tion of the salts (this chapter pp. 431 and 432), we always 

 find the sodic carbonate relatively decreasing as the surface 

 is approached. 



Thus, also, is explained the fact that sodium carbonate is 

 formed more abundantly toward the center of the root system 

 of alkali plants, such as the greasewood, beneath which the soil 

 is always more abundantly charged with " black alkali " than 

 is the surrounding earth. 



Good aeration of the soil mass, then, is essential in main- 

 taining the neutralization of the " black alkali '' soils brought 

 about by the use of gypsum (land plaster). 



Inverse Ratios of Alkali Carbonates and Sulfates. Ac- 

 cording to the above considerations, it is not surprising that 

 we should often find an apparent inverse ratio between the 

 alkali sulfates and carbonates in soils so closely adjacent that 

 their salts must be presumed to be similar in composition. A 

 striking example is shown in fig. 70, in which this inverse ratio 

 becomes apparent four times in succession in one and the same 

 soil profile. While this inference is plain on the face of the 

 diagram, it is not quite easy to explain in detail how this alter- 

 nation came about from the condition observed two months 

 previously. Most probably it was caused by corresponding 

 alternations of weather, in which short, warm spring showers 

 alternated with similarly brief periods of drying north winds; 



