EDAPHIC OR SOIL FACTORS: CHEMICAL ' 205 



Determination of Soda. — A 50-g. sample of the soil is boiled with 500 

 cc. distilled water, filtered, and diluted to 500 cc. This solution is 

 titrated with tenth-normal HCl, with methyl orange as indicator 

 (Na2C03 + 2HC1 = 2NaCl + H2CO3). The sodium content may 

 be calculated from the amount of tenth-normal HCl used in the 

 displacement of the carbonic acid. 



Vegetation of Soda Soils. — In his studies of the halophytic vegetation 

 of sodium soils in the Hungarian lowlands Bernatsky gives an account 

 of the plant communities. Kerner in his work on the plant life of the 

 Danube countries (1863) had already outlined the salt vegetation 

 of the great Hungarian Alfold. In large areas Statice gmelini pre- 

 dominates. Moist sandy saline soils are characterized by great masses 

 of Achillea crustata along with Aster pannonicus, Scorzonera parvi flora, 

 Erythraea linarifolia, Carex divisa, and other salt-favoring species.. 

 Deeper depressions in the soil with high sodium content, often plantless 

 in the center, are surrounded by a belt of dark reddish-brown Kochias 

 and Salicornias, adjacent to the ashy-gray Atriplex and Artemisia 

 salina belt. The companion floras of these communities are character- 

 istic and richly varied, containing Crypsis aculeata, C. schoenoides, 

 C. alopecuroides, Pholiurus pannonicus, Cyperus pannonicus, Lepidium 

 cartilagineum, L. ruderale, and L. perfoliatum. The osmotic concentra- 

 tion of the soil solution in the Hungarian sodium soils has been 

 examined by Stocker (1930). All sodium plants are able to put out 

 new absorbing roots in a few hours, as soon as the soil is sufficiently 

 moistened, but these roots dry up again as soon as the osmotic con- 

 centration goes above 28 atmospheres. 



Gypsum swamps of large extent occur in Australia, according to 

 Osborn (1925). In floristic composition they resemble those of 

 sodium chloride and sodium sulphate soils. The identical halophytic 

 genera which are spread over the European, north African, and central 

 Asiatic salt regions are also found in Australia. The Mesembryan- 

 themum australe association of the gypsum swamps of Flinders island 

 are surrounded by the grass Lepturus incurvatus, which is also widely 

 distributed upon the Mediterranean saline soils 



All saline soil solutions have high osmotic concentrations. The 

 osmotic value of the soil solution often acts as a limiting factor quite 

 aside from its specific composition. This makes quantitative deter- 

 minations of decided ecological value, and since such determinations 

 of the concentration of the soil water may be made easily and 

 quickly, they are desirable parts of any ecological investigation. If 

 possible, they should extend through the whole vegetative period or, 

 better, throughout the entire year. They can be made with the 



