394 



Mr. C. H. Martin. 



[May 10, 



It must, I think, be clear that on the surface of ordinary soils practically 

 any form of protozoan may be found either in the encysted or in the active 

 condition. If small surface pools are formed on well-manured soil it is 

 obvious that a large number of protozoa, including the larger ciliates, may 

 thrive. The question of the ciliates has already been dealt with by 

 Mr. Goodey,* who was, as far as I am aware, the first to describe some 

 of the protozoa of sick soil. But it seemed to me that, in spite of his paper, 

 much remained to be done, particularly in connection with the smaller 

 amoebae and the flagellates, which must, I believe, play the most important 

 rdle in the causation of this phenomenon. It seems probable a priori that the 

 most common limiting factor as regards the activity of protozoa in the soil is 

 the average quantity of water. It is generally assumed that in a fairly dry 

 soil — that is, a soil that feels quite dry to the touch — there is 20 per cent, of 

 water by volume ; but as this water is largely present in the soil as a 

 surface-film on the small soil particles, it is evident that the protozoa 

 capable of leading a trophic life under these conditions must be either of 

 a very small size or else must have developed some particular protective 

 mechanism against desiccation. It seems to me that there are two possi- 

 bilities for a protozoon that is going to do any marked harm in a soil during 

 a year (on the assumption that Eussel and Hutchison's hypothesis that soil- 

 sickness is due to the ingestion of soil-bacteria by protozoa) — either the 

 protozoon must be satisfied with a very low percentage of water in the soil 

 during its trophic life, or else it must have a capacity of readily encysting, 

 together with the capacity of reproducing with enormous rapidity as soon as 

 the soil becomes saturated with the necessary amount of moisture. It is 

 quite possible that all the harmful protozoa will be found to possess both 

 the above faculties to a certain degree — probably coupled, if they are to 

 make the best use of the water of the surface-film, with a small size. In the 

 case, however, of the soils of sewage farms, which are frequently saturated 

 with water, any of the protozoa, even the larger ciliates, would be capable of 

 acting with a certain amount of efficiency as a check upon bacterial life. 

 In order to limit the problem, I decided to examine a soil which, though 

 probably never very dry, was yet never in the waterlogged condition of the 

 soil on a sewage farm. Dr. Eussel was kind enough to send me samples of 

 sick soils from three different sources, and of these I have devoted most of 

 my attention to the sample derived from a sick tomato bed. 



For the purpose of separation of the organisms I used various types of agar- 

 plate, and as much still remains to be done from the cultural point of view, I 

 have decided to publish the full details of my methods in the later paper. It 

 * ' Eoy. Soc. Proc.,' 1911, B, vol. 84, p. 165. 



