CARBON AND NITROGEN CYCLES IN THE SOIL 215 



Protein 



AmirwAcids 



NH3 Hydroxy Acids 

 MtrUes , CalcimrL Jolts 

 ^'^',""' mites c}co, 



Carbohydrates 

 Cellulose 



1 r 



other Coinpounds 

 Etunus " 



COg 



a>. 



Oils Waxes 





Acids ' 

 CalcuuTLSalts 



Co. CO, 



iindecomposed 



CO, 



There is good reason to suppose that all of these changes are 

 effected by micro-organisms, but the evidence is by no means 

 conclusive. The obvious test of working with sterilised soil is 

 out of the question, because soil can only be sterilised by 

 drastic methods that wholly change its character. The fact 

 that antiseptics put an end to most of the reactions always 

 used to be regarded as suflficient proof of their biological nature, 

 but this argument has lost much of its force since Bredig and 

 others^ have shown that indisputably dead materials like 

 spongy platinum partially lose their power of bringing about 

 chemical changes when treated with antiseptics. Now the 

 soil is a spongy mass, measurably radio-active (p. 89), con- 

 taining numerous colloidal bodies conceivably capable of act- 

 ing as catalysts, and it is possible to imagine a series of 

 catalysts that would bring about all the know^ changes and 

 be put out of action by antiseptics. Hypotheses of this nature 

 have indeed been put forward from time to time and reactions 

 have been discovered which cannot be attributed to micro- 

 organisms. Cowie has shown ^ that the first stage of the de- 

 composition of cyanamide in the soil, the formation of urea, 

 is not due to organisms, but to some mineral constituent.^ 



pears extremely rapid to the agricultural chemist ; changes such as nitrification, 

 for which he is accustomed to allow days or even weeks, being brought about 

 in two or three hours even at the low temperature. 



For the decomposition of fats, see O. Rahn, Centr. Bak. Par., Abt. II., 1906, 

 15, 52-61 ; 422-429. 



^Cf. Bredig and Ikeda, Zeit. Physikal Chem., igor, xxxvii., 1-68. 



"^Journ. Agric. Sci., 1920, 10: see also Lohnis, Zeitsch.f. Gdrungsphysiol, 

 1914, 5, 16-25. 



* For a study of solid catalysts see E, F, Armstrong and Hilditch, Proc. Roy, 

 §oc., 1919, 96 A, 137. 



