204 PLANT SOCIOLOGY 



Soda Soils. — In less arid regions, in the northern part of the black- 

 earth zone of Russia, in Rumania, Hungary, also in the United States 

 (California) and Central Asia, soda soils rich in carbonic acid, take the 

 place of sulphate soils. The principal soluble salt is sodium carbonate, 



NasCOs. 



In contrast to the sulphate soils which are due to climatic conditions 

 it seems that soda soils are due to rather local edaphic conditions 

 occurring in medium to semiarid climates.^ 



According to investigations in California by Hilgard (1914) and in 

 Hungary by Treitz (1908), these soils are found only upon loess and 

 marl.- The deciding factor for their formation is, according to Treitz, 

 whether or not precipitation water drains off from the soil. If the 

 water does not drain off, the alkali salts collect as a residue in the soil 

 depressions. When the sulphates and chlorides in solution penetrate 

 the porous Hme soil (sodium formation is possible only upon soils rich in 

 lime), the calcium goes into solution as bicarbonate; this reacts with 

 the sodium salts forming alkahne salts of carbonic acid, while the lime 

 unites with the sulphuric and hydrochloric acids. 



During the rainy season the sodium carbonate dissolved in the soil water 

 passes from the upper layers of soil to the lower. Here it meets with crystal- 

 lized gypsum and is changed to sodium sulphate. When the dry season 

 comes on, the soil moisture rises toward the surface again. On reaching the 

 calcium carbonate resulting from the former reaction, the sodium sulphate 

 changes back to carbonate and rises to the uppermost layers of the soil 

 [Treitz, 1908, p. 119]. 



This salt migration is responsible for the fact that the soda soil is 

 free from sodium in the spring and rich in sodium sulphate in the 

 deeper layers, while in the fall sodium is abundant in the upper 

 stratum and but Uttle sodium sulphate remains deeper in the soil. 

 Soda soils (Hungarian szek) contain the halides in the unflocculated 

 form and are therefore finely divided, densely packed, and often crust- 

 like. Owing to the dispersion of organic material the soil solution is 

 colored black. In North America these soils are called "black-alkali" 

 land, in contrast to the "white-alkali" land (chloride and sodium sul- 

 phate soils) . Thus soda soils are distinguished not only chemically but 

 also physically from the other saHne soils which have a coarser and 

 more granular structure. 



1 In desert regions soda appears only with increased soil moisture, especially 

 in old stream beds and in marshy places (see Dimo, 1925). 



2 The formation of the North America soda soils, according to Breazeale, is due 

 ia many cases to the presence of a lime crust in a sodium chloride soil {cf. also 

 Dorsey, 1906). 



