354 SCIENCE PROGRESS 



Boussingault, in 1859, had recognised the existence of a "myco- 

 dermic vegetation . . . not always visible to the naked eye, the 

 progress of which must be followed by aid of a microscope," 

 there was no suspicion that this " vegetation " was in any way 

 connected with soil fertility. Not till the nature of putre- 

 faction was elucidated and the old doctrine of spontaneous 

 generation finally disproved, was it possible to make much 

 advance. Soil bacteriology may be said to date from 1878, 

 when Schloesing and Miintz observed a delay of twenty days 

 in the commencement of nitrification in an artificial soil in 

 contact with sewage, and argued that nitrification must there- 

 fore be a biological, not a chemical, process. The new hypothesis 

 was put beyond question by Warington and in course of time 

 the suspected organisms were actually isolated. A vast amount 

 of subsequent work has shown that the decomposition of plant 

 remains in the soil is mainly the work of micro-organisms and 

 has also revealed to some extent the broad outlines of the 

 change. 



It is necessary at the outset to appreciate the conditions 

 under which decomposition goes on in the soil. The greater 

 part of the soil is simply inert mineral matter : the decompos- 

 able part is only small ; normally there is not more water, 

 frequently there is less, than can be held by surface forces on 

 the particles. In a not unusual case the soil consists of about 

 80 per cent, of inert mineral matter, 15 per cent, of water, and 

 5 per cent, of organic matter, only part of which, however, can 

 be readily decomposed. The top six inches of soil thus con- 

 stituted is inhabited by a teeming population of the most varied 

 kind, from large earthworms down to organisms only 0*5 ^ in 

 length. Bacterial counts show that some millions are present 

 in each gram of soil. The micro-organisms show great diversity 

 in their food requirements, their mode of life and in the way 

 they are influenced by external conditions. All are competing 

 in the struggle for existence ; multiplying with enormous rapidity 

 whenever the conditions are favourable, disappearing equally 

 quickly when they are unfavourable or changing into spores 

 which may lie dormant during long periods and yet revive as 

 soon as the conditions are again suitable. In the struggle, no 

 one species exterminates the rest ; instead the different forms 

 seem to settle down to a rough kind of equilibrium, each being 

 hampered by others yet each surviving in some degree ; ap- 



