EDAPHIC OR SOIL FACTORS: CHEMICAL 203 



Solfataras, Fwnaroles. — Whereas the halophytic vegetation of 

 deserts and steppes consists chiefly of xerophytes, the plant communi- 

 ties of solfataras and fumaroles show a mesophytic character through- 

 out. The vegetation of the sulphur springs of Java has been given 

 much attention, even to the structure of tissues. There the soil surface 

 is covered with a yellow and whitish coating of sulphur and sulphur 

 compounds. The steaming springs are rich in alum. The beautiful 

 fern, Pteris incisa, nowhere thrives better than upon these soft sul- 

 phurous soils; Polypodium vulcanicum even ventures upon rocks which 

 are occasionally flooded by water reaching a temperature of TS^C. 

 According to Holtermann (1907, p. 78), the dominant shrubs, Agapetes 

 vulgaris and Rhododendron retusum, are completely covered with a 

 yellowish precipitate from the sulphurous fumes. In spite of it, how- 

 ever, the leaves retain a rich green color, and even in the warm mud, 

 surrounded with boiling solfataras, the shrubs bloom and bear fruit. 

 The leaf anatomy of all the species examined is remarkable for the 

 absence of xerophytic structures. 



Sulphur Formation by the Action of Bacteria. — The sulphur bacteria 

 (purple bacteria, Beggiatoa, Thiothrix, Thiovolum, Thiophysa spp., etc.) 

 which oxidize hydrogen sulphide are widely distributed in nature. 

 Their communities live chiefly in sulphur springs and on the shores of 

 brackish waters. The sulphuric acid resulting from oxidation of H2S 

 is neutrahzed by carbonates (mostly CaCOs) : (H2SO4 + CaCOs = 

 CaS04 + CO2 -}- H2O) and deposited as sulphate, in this case as 

 calcium sulphate. 



All sulphur bacteria require sulphur compounds for their develop- 

 ment. These are furnished as hydrogen sulphide by the decomposition 

 of plant and animal remains in water. Such bacteria are abundant in 

 quiet waters rich in sulphates, as described for the Danish coast and 

 the Kiel Fiord by Warming (1875) and Engler (1883). 



In the spring purple bacteria form communities upon the mud bot- 

 tom of the lagoons near Montpellier. When the inch-thick algal mat of 

 species of Ulva and Enteromorpha is removed, large areas of the moist 

 soil appear covered with an intensely wine-red, ill-smelling carpet of 

 bacteria. A survey of the sulphur bacteria, with extensive references, 

 is given by Diiggeli (1919). 



Determination of Sulphates. — A soil extract is made with slightly 

 acidulated water (HCl 1 per cent). In the filtrate the sulphate is 

 precipitated with barium chloride, BaCl2. The barium sulphate, 

 BaS04, is washed carefully and dried. Its weight, multiplied by 

 0.4114, gives the amount of sulphuric acid: (lg.BaS04 = 0.4115g.SO4) 

 (see also Wiegner, 1926, p. 39). 



