442 Observations on Soil rrotozoa 



attempt to regard the action between protozoa and soil particles as one 

 of adsorption is rather hopeless, and the fact that experiment negatives 

 such a view is not a matter for surprise. 



A further point of interest at present inexphcable is that so few 

 organisms are retained by the soil. Assuming the approximate area of 

 1 gram of Rothamsted soil to be 2500 sq. cm. this figure is much larger, 

 than the total area of the amoebae and flagellates retained by the soil, 

 which is only 4-2 sq. cm. approximately. In the case of fine and coarse 

 sand the area covered by the retained organisms is much larger, though 

 still below what might have been expected. 



Examination of the five columns in Table IV shows that as the 

 number of particles in the material increases so also does the number of 

 organisms taken up. It is remarkable, however, that fine sand proves 

 so effective as compared with coarse sand, and that ignited soil has a 

 capacity so nearly equal to that of the untreated and partially sterilised 

 soil. The particles of fine sand appear to be of sizes most suitable for 

 retaining the organisms. With ignited soil the power of retention is 

 almost as great as is that of untreated soil, thus indicating, under the 

 conditions of these experiments, that the effective agent is the surface 

 area of the particles irrespective of their colloidal properties. 



Part I of this paper showed that the dilution and direct methods are 

 comparable. It is safe then -to assume that the number of protozoa 

 found per gram of soil by the dilution method probably represents 

 fairly accurately the actual numbers in the soil sample. Since various 

 observers have shown that the number of amoebae and flagellates 

 usually present in the soil is between 10,000 and 100,000 per gram, it is 

 evident that the number of protozoa in an average sample of soil is 

 far less in number than the soil is capable of retaining. Russell and 

 Golding(5) found numerous protozoa in sewage sick soils and by the use of 

 the centrifuge they were able to obtain some of the active forms free from 

 the soil particles. Probably the conditions were such that excessive 

 reproduction of the protozoa occurred until the numbers were greater 

 than the retaining power of the soil. Protozoa would then be found lying 

 free from soil particles and would be acted upon by the centrifuge. 

 Further investigations on these lines are in progress. 



Part II of this paper demonstrates that the protozoa are normally 

 resident on soil particles, therefore their environment may be of a different 

 nature from that sometimes assumed. Russell and Appleyard(6) showed 

 that the "free " air of the soil was approximately that of the atmosphere, 

 but that there was also a second atmosphere dissolved in the colloidal 



