D. W. Cutler 441 



for retaining amoebae and flagellates is practically the same as the ratio 

 of their capacities to retain ciliates: 



Ratio of 



Coarse Sand Fine Sand Soil to Soil to 



to Soil to Soil Ignited Soil Clay 



Amoebae and Flagellates 1 : (i-T 1:1-6 1:1 OO 1 : I ••■» 



aiiates 1:0-8 1:1-5 1:1-04 1:1-6 



The ratio of the mean diameter of the amoebae or flagellates to 

 that of ciliates is as 1 : 5, while the ratio of the mean voliune of the 

 amoebae or flagellates to that of the ciliates is as 1 : 5^. On the other 

 hand the ratio of the holding power of the various substances used is 

 for ciliates and amoebae or flagellates as 5 : 1 approximately. Thus 

 the ratio of the retaining powers of the various substances is inversely 

 proportional to the ratio of the diameter of the protozoa and to the 

 cube root of their volumes. Some relationship between these variants 

 seems probable, but at present it has not been discovered. 



Discussion. 



The foregoing results demonstrate that the factors governing the 

 relation between soil protozoa and soil particles are largely physico- 

 chemical and primarily of the nature of surface action. As the size of the 

 particles diminishes so the number of protozoa retained increases, till 

 finally 1 gram of clay withdraws 2,-500,000 flagellates and amoebae from 

 1 c.c. of the suspension. Different types of soil probably differ in their 

 capacities according as their content of sand or clay was high or low, 

 for it has been shown that the results are the same if the suspension is 

 allowed to filter through the soil as would occur in a field. 



The surface action, however, between the protozoa and the soil 

 particles appears to difEer from ordinary adsorption. The action is hnear 

 up to the point when a suspension is used of a strength less than the 

 retention capacity of the substance, then complete withdrawal of the 

 organisms from the suspension takes place. This is in sharp contrast 

 with adsorption, which is never complete. Also there is no similarity 

 between a typical adsorption curve and those given in fig. 1. Nor could 

 any be expected. Rothamsted soil is estimated to contain some 12,000 

 milhon particles per gram, possessing an area of the order of 2,500 sq. 

 centimetres: 18 per cent, is clay with particles oi 2 ju, downwards; 

 53 per cent, is silt with particles of diameter 25-6 jx. The average 

 diameter of the protozoa is much greater than that of the clay particles 

 and equal or only sHghtly less than that of the silt particles. Thus any 



