BACTERIA AND PROTOZOA 29 



power of starting with the inert nitrogen of the soil 

 air and of changing it into nitrogenous plant food. 

 The rotted manure gives it the food of which -it 

 stands in need, and under favourable conditions it 

 is able to go on steadily adding to the store of nitrog- 

 enous food material in the soil. 



Such in crudest outline is the function of the soil 

 bacteria and the way in which they lead to an 

 increase in soil fertility. In the chapters immedi- 

 ately following I shall endeavour to describe their 

 work in greater detail, and to show how it is possible 

 for man successfully to co-operate with them. A 

 word, however, in conclusion of this chapter must 

 be written of the protozoa, for though they are not 

 concerned with the general argument of this book, 

 they bear an important relation to the soil bacteria 

 and seriously affect their welfare. 



The protozoon is not a bacterium, but a minute 

 animal requiring the magnification of a strong micro- 

 scope lens to be seen. Small though he is, he is as 

 terrible and monstrous a foe to the bacteria as the 

 vast swamp monsters must have proved to emerging 

 man in the Eocene times. Looked at in water under 

 the microscope he is seen creating in the medium 

 whirlpools against which the hapless bacteria are 

 powerless to contend, and drawing them defenceless 

 into his body. Recent experiments at Rothamsted 

 have shown that these animalculae in certain con- 

 ditions may seriously check the bacteria of the soil, 

 and may gravely affect the health of plants, especially 

 in rich, ^ highly-manured soil. Experimentally in 

 horticulture it has proved practicable to get rid of 



