10 



their number continues to rise till it exceeds that present in the 

 original soil. Simultaneously there is a marked increase in the 

 ammonia. It is evident that the soil has been improved by the 

 treatment, and this improvement is found to be permanent. They 

 argued, tnerefore, that there must be in the soil some factor that 

 limits the activity of the bacteria, and since this limiting factor is 

 abolished by the treatment it cannot be the lack of anything, but 

 must be the presence of something which is affected by the treat- 

 ment. They came to the conclusion that some living tilings exist- 

 ing in the soil were detrimental to the bacteria. They proceeded to 

 search for these living things in cultures of untreated soil, and 

 they discovered the presence of minute animals known as 

 Protozoa. The investigation is a very difficult one, and these 

 difficulties are only now being partially overcome. 



Stimulated by these preliminary observations, several biologists 

 proceeded to examine the fauna of soils, and a series of researches 

 published in 1911, and still going on, has already resulted in the 

 discovery of quite a large variety of much larger organisms, though, 

 of course, they are still of microscopic size.t These are unicellular 

 animals or Protozoa, known to us as Amoebas, Flagellates, and 

 Ciliates, such as exist in great numbers in pond water and in sewage. 

 \ They are now known to occur in considerable numbers in the soil, 

 being especially numerous in "sick soil," where there may be as 

 many as thousands to a gramme, which is, indeed, but a low number 

 . compared with the bacteria, which occur in millions per gramme of 

 soil. It is these Protozoa that feed upoit the bacteria, and hence 

 the cessation of the important nitrifying process. When the soil 

 is treated by partial sterilisation or by antiseptics, the Protozoa are 

 killed, while the more hardy bacteria may escape, but after the re- 

 moval of the toluene, or after sterilisation has ceased, the bacteria 

 renew their activity and multiply rapidly. 



This then is an important additional piece of knowledge allowing 

 soils to be effectively treated when they become sick. Until this 

 discovery was made any attempt to effect a remedy could only be a 

 matter of working in the dark. 



It is found, indeed, that burnt lime, quick lime, spread on the 

 soil has the same effect as partial sterilisation. It kills the Proto- 

 zoa, whereas ordinary carbonate of lime does not. 



tGoodey, "A Contribution to our Knowledge of the Protozoa of the Soil," 

 Proc. Roy, Soc., vol. 84, B., 1911, p. 165. 



Martin, "A Note on the Protozoa of Sick Soils, with some account of the 

 Life-Cycle of a Flagellate Monad," ibid., vol. 85, B., 1912, p. 393. 



Thornton and Smith, "On the nutritive conditions determining the growth of 

 certain Freshwater and Soil Protista," ibid., vol. 88, B., 1914, p. 151. 



Russell, "Soil Protozoa and Soil Bacteria," ibid., vol. 89, B., 1915, p. 76. 



Goodey, "Further Observations on Protozoa in Relation to Soil Bacteria " 

 ibid., vol. 8*9, B., 1916, p. 297. 



Martin and Lewis,' "Some Notes on Soil Protozoa," Phil. Trans R Soc vol 

 205, B., 1914, p. 77. 



JHutchiason, "Journ. Agric. Science." 



