THE MICRO-ORGANIC POPULATION OF THE SOIL 293 



teria are in inverse relationship when one is high the other 

 is low. 1 



This relationship appears always to hold when counts are 

 taken at short intervals. Since amoebae normally feed on bac- 

 teria and do not live saprophytically they seem indicated as 

 suppressors of bacterial numbers ; they are, however, probably 

 selective in their food, and it is possible that all soil bacteria 

 are not equally affected by them. 



The effect of flagellates in the soil is quite unknown. They 

 do not necessarily feed solely on bacteria, but can and frequently 

 do utilise other substances, including amino-acids. 2 Thus it 

 would appear that they must effect some changes in the soil. 

 Their large numbers and rapid fluctuations suggest an im- 

 portant part in soil economy which at present is unknown. 



If protozoa are detrimental to soil bacteria it ought to be 

 possible to reduce bacterial numbers in protozoa-free soil by 

 the introduction of a protozoan fauna. Considerable import- 

 ance is attached by Sherman and others to the fact that this 

 fundamental experiment has not yet been satisfactorily carried 

 out, or at any rate not more than once or twice, only A. Cun- 

 ningham 3 and T. Goody (iio) having reported decreases in 

 numbers. 



The experiment is admittedly crucial, but it is very diffi- 

 cult and is not likely to succeed until more is known of the 

 life histories and physiology of the particular organisms con- 

 cerned, and of the conditions necessary for their growth in 

 soil. Cutler's results (Fig. 29) are difficult to explain in any 

 other way except by supposing that active amcebae are detri- 

 mental to bacterial numbers. 



The present position in the controversy that has arisen may 



1 The relationship thus resembles that obtaining in water : see Otto Hunte- 

 mUller, Vernichtung der Bakterien in Wasser durch Protozoen (Arch.f. Hyg., 1905, 

 liv., 89-100) ; Fehrs, Die Beeinflussung der Lebensdauer von Krankheitskeimen in 

 Wasser durch Protozoen (Hyg. Rundschau, 1906, xvi.) ; P. Th. Muller, Ueber die 

 Rolle der Protozoen bei der Selbst-reinigung stehenden Wassers (Arch.f. Hyg., 

 1912, 75). 



2 See H. G. Thornton and Geoffrey Smith, On Certain Soil Flagellates 

 (Proc. Roy. Soc., 1914, 88 B, 151-165). 



3 Journ. Agric. Sci., 1915, 7, 49-74. 



