IV. THE KEACTIONS OCCURRING IN SOILS 61 



The organisms concerned consist chiefly of protozoa and amoebae 

 (belonging to the animal kingdom), fungi, including moulds and yeast, 

 bacteria and algae. 



Protozoa and amoebae, according to Eussell and Hutchinson J are 

 always present in soils and live on bacteria, thus keeping down the 

 numbers of the latter. The fertility of a soil, according to this theory, 

 depends mainly upon the number of ammonia-producing bacteria 

 present, and this, in turn, depends upon the activity of the protozoa and 

 amoebae for which the bacteria serve as food. Any cause, e.g., sterili- 

 sation by heat or by antiseptics, which destroys, or diminishes the 

 number of the protozoa, enables the bacteria to increase rapidly and 

 thus to accelerate the production of ammonia from the nitrogenous or- 

 ganic matter of the soil. The investigators found that in a Eothamsted 

 soil containing about 7,000,000 micro-organisms per gramme, heating 

 reduced the number to about 400 per gramme, but after moistening 

 and keeping for four days, the bacteria became as numerous as ever, 

 and in a few more days became far more numerous than they were 

 originally. At the same time, the rate of ammonia production in the 

 soil enormously increased, though the conversion of ammonia into ni- 

 trites and nitrates practically ceased. 



The enhanced fertility of soils induced by sterilisation is therefore 

 due to the destruction of protozoa and amoebae and the survival (doubt- 

 less due to spore formation) of the ammonia-producing bacteria, which , 

 when again placed under conditions suited to active growth, increase 

 at an enormous rate, being freed from the destructive influence of the 

 protozoa, which in the original soil limited their number. 



The fungi and yeasts act upon certain kinds of organic matter in 

 the soil, the former using this material to build up their own structure 

 and then, by their 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. 2 



The vital processes, too, ot many of the moulds appear to be con- 

 nected with important changes in nitrogenous organic matter (e.g., the 

 formation of ammonium carbonate from proteids). 



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, spirilla or corkscrew- 

 like, micrococci or spherical, organisms. Their size is very minute, be- 

 ing about T ^V(T f a millimetre in diameter and rarely exceeding T (^<y 

 of a millimetre 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 resting 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 germin- 

 ating under favourable conditions. 



1 Jour. Agric. Soi., 1909, 3, 111. 



2 Lawes, Gilbert and Waiington, Jour. Chem. Soc., 1883, Trans., 208. 



