78 THE EEACTIONS OCCURRING IN SOILS. IV. 



details as to the methods of investigating their nature and 

 functions the student is referred to a manual of bacteriology, 

 a subject which now has an extensive literature. 



The micro-organisms in a soil include moulds, yeasts, and 

 bacteria, and in addition there are minute plants (e.g., algae), 

 which grow, probably, only on the surface. These latter 

 appear to be able to assimilate carbon and nitrogen from the 

 air and build up organic compounds from inorganic materials 

 (see p. 48, Chap. III.). 



The fungi and yeasts act upon certain kinds of organic 

 matter in the soil, the former using this material to build up 

 its own structure and and then, by its decay, leaving again a 

 residue which in many cases appears to be more susceptible to 

 nitrification than the original. It is to such an action of a 

 fungus, spreading outwards from a starting point, that the 

 existence of those richer coloured and more luxuriant circles of 

 grass in pasture fields known as " fairy rings" is due." 



The vital processes, too, of many of the moulds appear to 

 be connected with important changes in nitrogenous organic 

 matter (e.g., the formation of ammonium carbonate from pro- 

 teids). 



But most interesting, perhaps, are the minutest forms of 

 life known as bacteria. These bodies are of various external 

 forms and are often classified into some four or five groups 

 according to their characteristic appearance. Thus there are 

 bacilli or rod-like organisms, spirilla or corkscrew-like, micro- 

 cocci or spherical. Their size is very minute, being about 

 TO^O f a mm - i n diameter and rarely exceeding T Q%O of a mm. 

 in length. Bacteria multiply by simple fission, but many 

 forms have the power, at intervals, of reproducing themselves 

 in another manner, viz., by spore formation. Spores are rest- 

 ing states of existence and can resist treatment which would, 

 at once, kill the active form of the bacterium. For example, 

 they may be dried and some even heated to 100 C. without 

 destroying their power of germinating under favourable con- 

 ditions. 



Ordinary soils contain large numbers of different bacteria, 

 some fulfilling useful functions in agriculture, some being 



* Lawes, Gilbert, and Warington, J.C.S. 1883, Trans. 208. 



