Dec. 13, 1915 Activity of Soil Protozoa 487 



small ciliates are seen to excyst in from six to eight hours after the immer- 

 sion of the cysts in water. From the limited amount of study given to 

 this point no conclusive statement as to the relative length of time required 

 for the excystment of soil protozoa can be made. Nevertheless, the writer 

 is of the opinion that under normal conditions protozoa excyst seldom, if 

 at all, in as little as two minutes. There may be cases, however, as where 

 the protective cyst is partially ruptured either by mechanical means or 

 otherwise or where the moisture conditions are almost favorable enough 

 for excystment, in which the organisms will become active in less than two 

 minutes; but under ordinary normal conditions it seems doubtful from 

 the examinations already made whether they can become active in this 

 period of time at 22° to 24°. The indications (Table IV) are that excyst- 

 ment goes on more rapidly at higher temperatures. In all probability 

 the original moisture content of the soil plays a part in determining the 

 length of time which must elapse before the organisms become active. 

 Likewise, different types of protozoa will prefer different conditions (i) and 

 may excyst sooner at one temperature than at another. Further study 

 on this point will be made. 



SUMMARY 



Under the conditions recorded in this paper the following observations 

 as to the activity of soil protozoa seem to be justified : 



(i) Under ordinary greenhouse conditions small ciliates, flagellates, and 

 amebiE are active in some soils, but their presence is very limited. 



(2) Active protozoa (small ciliates, large ciliates, flagellates, and 

 ameb^e) do not seem to be present in field soils with a normal moisture 

 content and even when the moisture content is slightly supernormal, 

 and, hence, they would not be a limiting factor in the soil. 



(3) All field soils contain cysts of protozoa the organisms of which 

 become active when conditions become favorable. 



(4) The moisture content of the soil is the primary influencing factor 

 which determines the presence or absence of the active protozoa in the 

 soil, while the temperature, the presence of organic matter, and the 

 physical properties of the soil are secondary factors. 



(5) Soon after standing water is accumulated, as after a heavy rain, 

 some protozoa will excyst and be active as long as the moisture content 

 is favorable. Active protozoa seem to be always present in free standing 

 soil water. 



(6) Active protozoa are present in water-logged soils at constant and 

 variable temperatures. 



(7) Under normal conditions it would seem that protozoa can not 

 excyst in 2 minutes. Small ciliates can excyst in i to 2 hours at 22° to 

 24° C. ; at the same temperature flagellates can excyst in 6 to 8 hours 

 and large ciliates can excyst in 40 hours. 



