The Soil as a Habitat for Life ' 



By Sir John Russell 



Late Director of the Rothamsted Experiment Station 

 Vice President of the British Royal Agricultural Society 



[With 5 plates] 



The dark and seemingly inert structure of a lump of soil would not 

 at first sight seem to constitute a likely habitat for life. Even in the 

 1880's and 1890's, when bacteria were fomid as regular inhabitants 

 playing an important part in the nutrition of plants, nothing larger 

 was supposed capable of existence in the soil. Later investigations at 

 Rothamsted, however, showed that active protozoa were present as 

 well as bacteria, and in 1919 a team was set up there to study these 

 and other organisms also found — algae, fungi, and small inverte- 

 brates — not simply as individual groups, but as a mutually inter- 

 dependent population.^ Lack of resources brought the scheme to 

 an end, but work on individual groups continued, and has recently 

 been greatly extended, especially on the small invertebrates. 



ORGANISMS IN THE SOIL 



As usual, every advance shows that the field is much wider than 

 had been expected. The old estimates of the numbers of active bac- 

 teria in farm soil at Rothamsted ranged from about 3 to 10 million 

 per gram ; modern estimates are 3,000 million or more. It is difficult 

 to visualize either the size or the numbers of bacteria in the soil. 

 One of the best illustrations I know of the size of bacteria was given 

 by S. G. Paine when he estimated that a quarter of a million of 

 average size could sit on the period at the end of this sentence. Esti- 

 mates of their weight per acre to a depth of 6 inches have ranged 

 between 1,500 and 3,000 pounds at Rothamsted, and up to 4 tons on 

 a Swiss grass field — wliich is considerably greater than that of the 

 animals grazing on it. The numbers and variety of the soil animals 



1 Reprinted by permission, with some substitution of illustrations, from Discovery for 

 December 1961. 



s Russell, E. John (ed.), Microorganisms in the solL Longmans, 1923. 



449 



