82 



Soil Protozoa and Soil Bacteria. 



heat, antiseptics, or prolonged storage, other changes are produced besides the 

 destruction of the limiting factor and the protozoa. Some ammonia is formed 

 and the amount of soluble matter is increased — both evidence of a change in 

 the soil constituents — and within a few days after remoistening great numbers- 

 of bacteria and of their decomposition products accumulate. 

 Thus the two systems are : — 



(1) Untreated soil containing normal numbers of bacteria and protozoa. 



(2) Partially sterilised soil, changed somewhat, and containiDg abnormal 

 numbers of bacteria and accumulation of their products. 



It is obvious that the addition of protozoa to (2) does not make it equal to 

 (1), even if the added protozoan fauna were identical with that in (1) and had 

 an equal chance of growth. But Cuningham shows it has not, for he finds 

 that the development of protozoa in a medium containing exceptionally large 

 numbers of bacteria is considerably hindered. It is significant also that- 

 some 5 per cent, of untreated soil has to be added to partially sterilised soil 

 in order to reintroduce the factor detrimental to bacteria. Nor is the 

 apparently simple case of introducing a protozoan fauna into partially 

 sterilised soil much better. The protozoa are not obtained in pure culture 

 alone; they are added along with hay infusion and bacteria. Thus the two 

 systems are : — 



(1) Partially sterilised soil containing high numbers of bacteria. 



(2) Partially sterilised soil containing high numbers of bacteria + hay 

 infusion + added bacteria + protozoa. 



These considerations show that no clear issue is obtained between soiL 

 protozoa on the one hand and soil bacteria on the other. Goodey's failure to 

 observe any reduction in numbers in the circumstances cannot, therefore, be 

 taken to justify the conclusion that ciliates, amoebae, and flagellates do not- 

 limit the number of bacteria. Until more is known of the kinds of protozoa 

 occurring in the trophic state in the soil, and of their life-history in the soil, 

 it will not be possible to lay much stress on the negative results of 

 re-infections : on the other hand, Cuningham 's experiments indicate that 

 positive results may be looked for in the near future. 



