C. H. Martin and K. R. Lewin 117 



IV. Conclusions. 



It seems probable from the work that we have done up to the present 

 that there are always some free living protozoa present in a trophic state 

 in even relatively dry, poor soils. 



In manuring on ordinary soil with farmyard manure, a large 

 number of protozoa are introduced into that soil, and if the conditions 

 of culture are such as to necessitate a high water and a high manurial 

 content, the protozoa may well get the upper hand to such an extent 

 as to produce a well-marked deleterious effect on the crop, resulting 

 in the condition known as soil sickness (e.g., in cucumber beds, sewage 

 farms). 



The nature of the protozoan fauna seems to vary to a certain extent 

 with the soil under examination. It is probable that this is largely 

 due to actual difference in the fauna of different soils, but it may be 

 partially due to another factor. As is well known, if some soil is added 

 to a hay infusion or other suitable culture medium, the fauna shows a 

 tendency to rim in cycles {e.g., at first the donainant forms would be 

 found to be small flagellates; these are usually followed by larger 

 flagellates and amoebae, and these are succeeded by ciliates). It is 

 possible that such cycles may occur in the soil, and it is possible there- 

 fore that two soils with a similar water content may show quite different 

 active fauna, depending on the point of the animal cycle at which that 

 soil had arrived. The dominant protozoa found in a trophic state in 

 a soil may be the dominant form found in the cultures, as was probably 

 the case in some sick cucumber soils ; but it of course depends on the 

 suitability of the medium, and the culture method adopted. It is 

 probable that the richer the soil and the higher the water content at 

 the time of examination, the greater the probability of the dominant 

 culture form being the dominant trophic form in the fresh soil. A 

 possible exception to this rule is furnished by the thecamoebae, which 

 usually only appear late under present cultural conditions. 



It will be seen that up to the present the dominant active fauna of 

 the soil, as shown by the fresh films, consists mostly of amoebae, thec- 

 amoebae and small flagellates. 



In this connection there is one point which requires further investi- 

 gation, and that is the frequent prevalence of relatively large flagellates 

 in soil cultures {e.g., Prowazelda and Copromonas), whereas in fresh 

 films the only flagellates found are very small monads. It may perhaps 

 be found that the Prowazekia are present in the trophic state only in 



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