314 Observations on Protozoa in Relation to Soil Bacteria. 



delicatula a member of the Proteomyxa, and also of the flagellates. The 

 (T -f 5 per cent. U) soil also showed evidence of the activity and multiplication 

 of certain large ciliates, whilst the (H + 5 per cent. U) did not. 



Conclusions. 



1. Protozoa, especially amoebae of the Umax group and other larger forms, 

 can lead an active existence and multiply in soil and exert a depressing effect 

 on bacterial numbers. 



2. It is probable that for a given soil a certain point must be reached in 

 protozoal numbers before the depression in bacterial numbers is caused. In 

 the soil under investigation this number appears to be approximately 30,000 

 Amoeba Umax per gramme. 



3. It appears to be necessary to add the protozoa to a treated soil in a 

 small quantity of untreated soil to ensure their having a suitable medium in 

 which to grow and multiply. Under these conditions it is shown that they 

 can increase in numbers and depress the numbers of bacteria. 



4. It does not appear to be possible to carry out mass inoculations of 

 protozoa into treated soil in such a way that they come into action and limit 

 bacterial activity, and the explanation advanced to account for this failure is 

 that the treated soil alone affords an unsuitable medium for the active trophic 

 existence of protozoa. 



