THE ORGANISMS OF THE SOIL 153 



action of plants much in the way in which Cohn has shown, 

 calcium carbonate is deposited in the Sprudelstein of Carlsbad. 1 



Bacteria have been studied principally from the standpoint 

 of disease in man and animals but recently the attention of 

 agriculturists has been directed to the nitrifying organisms. 

 The first step in the fixation of atmospheric nitrogen is accom- 

 plished by certain flagellate cells called Nitrosomonas, belonging 

 probably to the animal kingdom ; these are succeeded by 

 minute rod-like bacteria called Nitrobacteria, which oxidise the 

 product of the former into nitrates. These latter live principally, 

 or perhaps more properly should be described as having been 

 detected living, in the root nodules of clover, peas and similar 

 leguminosae, and have been called Rhizobium Icguminosum ; 

 they are occasionally found on the roots of forest trees and 

 it is now recognised that bacteria with similar functions live 

 free in the soil. Another nitrifying organism is the Azotobacter 

 chroococcus, which lives on the leaves of trees and causes leaf 

 mould to be so rich in nitrogenous compounds. 2 Some of the 

 bacteria and some of the ferments also have the power of 

 undoing the work of these nitrifying bacteria ; they denitrify 

 and liberate nitrogen from nitrates. The fact is familiar to 

 gardeners in the case of fresh stable manure, where the action 

 is brought about by denitrifying bacteria, whereas from rotted 

 manure the deleterious organisms are absent. 3 



Regarding the action of the blue-green algae in forming soil 

 Fritsch has accumulated a large amount of information. 4 In a 

 large tank at Nalande in Ceylon the first forms to secure a 

 foothold on the bare rock were found to be red-coloured 

 gelatinous species of the genera Gloeocapsa and Aphanocapsa ; 

 then, when a resting-place is secured, an adhesive species^ 

 Phormidium laminosum, grows upon it, covering large portions 

 of the rock surface with huge thin papery films. Tangled 

 filaments of Scytonema develop out of the Phormidium and tufts 

 of Tolypothrix succeed these. Treub, who visited Krakatoa after 



1 F. Cohn, Nates Jahrb. 1864, p. 580 ; see also W. H. Weed, The Formation 

 of Hot-Spring Deposits, Intermit. Congr. Geol., Compte Rendu, 5th Sess. 1893, 

 p. 360. 



2 A. D. Hall, Recent Developments in Agricultural Science, Addresses and 

 Papers, Brit, and S. Afr. Assoc. Adv. Sci. Johannesburg, 1905, vol. i. p. 103. 



3 R. Burri and A. Stutzer, Centr.f. Bakt. (2), vol. i. 1895, p. 442. 



* F. E. Fritsch, The Role of Algal Growth in the Colonisation of New Ground, 

 Ceogr. Jourti. vol. xxx. 1907, p. 531. 



