EDAFHIC OK SOIL FACTORS: CHEMICAL 191 



tussocks iron bacteria {Crenothrix polyspora, Chlamydothrix ferruginea, 

 etc.) live on the finely divided humus releasing iron hydroxide. This 

 product is precipitated as an intensely reddish-yellow, flaky substance, 

 while the moor water itself appears to be covered by an iridescent, 

 oily film. In a similar manner, it is believed, Crenothrix manganifera is 

 active in the precipitation of manganese. 



4. Zinc, Copper. — Soils which contain zinc, although of rather 

 limited extent, claim a few strictly characteristic elementary species 

 such as Viola calaminaria, which is related to Viola liitea, and several 

 elementary species of Thlaspi. According to Douin, a few liverworts 

 are favorable to heavy metals, especially copper, while of central 

 European species only Mielichhoferia iiitida is stated to be copper 

 favoring. In the Andes, however, according to Herzog, numerous 

 species of Mielichhoferia are said to exist as copper favorites. 



The vegetation on soils rich in zinc has recently been studied by 

 Libbert (1930) in the Harz and by Schwickerath (1931) near Aix-la- 

 Chapelle. In addition to the very specialized Viola calaminaria, the 

 latter lists Arineria elongata and Thlaspi calaminare and varieties of 

 Minuartia verna, Silene vulgaris, and Festuca ovina, as occupying the 

 thin sod (Violetum calaminariae) over extensive areas. The soil here 

 has always considerable zinc amounting to 0.78 per cent in the upper 

 horizons and 0.73 per cent in the lower. One slope at Breininger Berg 

 where the soil has 8.69 per cent of zinc is entirely bare of vegetation, 

 this amount of the metal being apparently quite toxic. 



5. Chloride, Sulphate, Soda (Salts of "Saline Soils").— In the 

 plant geography, as also in the agriculture and forestry, of coastal 

 lands, desert, and steppes "saline soils" play an important role. They 

 are in part climatic and in part edaphic and show accordingly different 

 physical and chemical compositions. 



The edaphically limited saline soils owe their origin chiefly to the 

 world's salt-storage basin of the seas. Consequently they are strongly 

 influenced by the chemical composition of the sea water, and they are 

 rich in sodium and chlorine. Of the salts dissolved in sea water about 

 80 per cent is sodium chloride. 



The climatically conditioned saline soils of deserts and desert 

 steppes are distinguished from edaphically conditioned saline soils by 

 greater dryness and by predominance of sulphates and carbonates, 

 especially the sulphates of sodium and magnesium (Na2S04,MgS04). 

 The amount of tabic salt, NaCl, falls off somewhat in desert soils, while 

 soda, Na2C'03, sometimes appears in large proportions. 



The floristic distinctions between sodium chloride, sodium sulphate, 

 and sodium carbonate soils are yet to be ascertained. No doubt, there 



