108 Met lit 11 Is for Exiiiii'iiKilinii of Soil Protozoa 



for tlie water content of the whole soil would give no indication of the 

 actual minimum quantity of water in wliich protozoa could remain 

 active. This difficnUv would he felt even if the soil were a honioi;eneous 

 mixture; but unfortunately this is far from being the case, and it is 

 certain that in a relatively dry soil the fragments of manure and of 

 decaying plant roots would hold a far larger amount of water tlian is 

 indicated by an ordiiiaiy determination of the water content, so that 

 if, for example, one kilogramme of soil contained 950 grammes of soil 

 particles and 50 grammes of decaying organic matter on which protozoa 

 W'ere flourishing, the figure given by the estimation of the amount of 

 water present in the soil would give no indication as to the actual 

 amount of water in the space where these protozoa were leading an 

 active life. 



Another important question is the difference between a coarse 

 grained and a fine grained soil with an equal percentage of water. It 

 would seem quite possible that an active protozoan fauna would be 

 found in the large water spaces in the former at a time when the latter 

 would exhibit no free forms. 



Further, when conditions in dift'erent soils are to be compared, it 

 is preferable from the biological point of view* to express water content 

 as percentage by true volume rather than as percentage by w^eight. 



As regards bacterial counts, all the points wliich have been urged 

 in connection with the heterogeneous nature of the soil carry here even 

 more weight. In the first place, it is probable that the bacteria are 

 concentrated in groups round decaying organic matter, and it has been 

 found in the examination of fresh films fiom tlie soil that the bacteria 

 are present as colonies, and are not scattered singly like currants througli 

 a cake. It is obvious that the bacterial count inust very largely depend 

 on the degree to which these colonies are broken up during the process 

 of dilution. It is well known, also, that the numbers obtained are 

 dependent upon the medium adopted, and on the conditions of culture. 



When the heterogeneity of the soil is taken into consideration, it 

 would seem impossible to hope for an accurate method for the estima- 

 tion of the active protozoa present in a soil. It is, however, possible 

 that practicable approximate methods may be devised, but before they 

 can be considered satisfactory as a basis for extended experiments, it is 

 very necessary that the range of their probable error should be known. 



Up to the present, the only method proposed for the enumeration 

 of the soil fauna is the dilution procedure described by Rahn (11). The 

 \Vork of Cunningliam and Li)hnis{:5) on the thermal death-point of the 



