Dec. 13, 1915 
Activity of Soil Protozoa 
487 
small dilates 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 0 to 24 0 . 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 (1) 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: 
(1) Under ordinary greenhouse conditions small ciliates, flagellates, and 
amebae are active in some soils, but their presence is very limited. 
(2) Active protozoa (small ciliates, large ciliates, flagellates, and 
amebae) 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 1 to 2 hours at 22 0 to 
24 0 C.; at the same temperature flagellates can excyst in 6 to 8 hours 
and large ciliates can excyst in 40 hours. 
