Soil Protozoa and Soil Bacteria. 



77 



complex mixture of easily soluble oxides of iron, aluminium, and silicon, 

 organic substances, nutrient salts, etc., and behaving physically like a colloid. 



About one-third to one-half of the volume of the soil consists of pores into 

 which air diffuses fairly readily, so that the percentage of oxygen is almost 

 the same as in the atmosphere, although that of carbon dioxide is higher, 

 ranging up to 1 per cent, instead of - 03 per cent. These pores also contain 

 water, the volume of which normally varies from 15 to 30 per cent, of the 

 total volume of the soil. This is mainly distributed in films over the 

 substances coating the mineral particles, and in proportion to those coatings 

 the volume of water is, of course, considerably greater than is here indicated. 



The soil is known to be inhabited by numbers of bacteria, eelworms, 

 Vermes and numerous other organisms of higher orders and visible dimensions ; 

 all these lead active lives. Recently it has been shown in the Eothamsted 

 Laboratory that a protozoan fauna also exists, some members at least of which 

 are leading a trophic existence. The investigations on the partial sterilisation 

 of soil indicate that the activity of these trophic forms is one of the factors 

 limiting the number of bacteria in the soil and consequently the amount of 

 decomposition they effect. It is this conclusion that is controverted by 

 Mr. Goodey in his recent paper. 



The experimental results leading up to this conclusion are as follows* : — 



1. Partial sterilisation of soil, i.e. heating to a temperature of 60° C. or 

 more, or treatment for a short time with vapours of antiseptics such as toluene, 

 causes first a fall then a rise in bacterial numbers. The rise sets in soon after 

 the antiseptic has been removed and the soil conditions are once more 

 favourable for bacterial development ; it goes on till the numbers considerably 

 exceed those present in the original soil. 



2. Simultaneously there is a marked increase in the rate of accumulation of 

 ammonia. This sets in as soon as the bacterial numbers begin to rise, and 

 the connection between the two quantities is normally so close as to indicate 

 a causal relationship ; the increased ammonia production is, therefore, 

 attributed to the increased numbers of bacteria. There is no disappearance 

 of nitrate ; the ammonia is formed from organic nitrogen compounds. 



3. The increase in bacterial numbers is the result of improvement in the 

 soil as a medium for bacterial growth and not an improvement in the bacterial 

 flora. Indeed the new flora per se is less able to attain high numbers than 

 the old. This is shown by the fact that the old flora when reintroduced into 

 partially sterilised soil attains higher numbers and effects more decomposition 

 than the new flora. Partially sterilised soil plus - 5 per cent, of untreated 



* The details are given in two papers by Russell and Hutchinson in ' Journ. Agric. 

 Sci.,' vol. 3, pp. 111-144 (1909), and vol. 5, pp. 152-221 (1913). 



