ioo SOIL CONDITIONS AND PLANT GROWTH 



Summary of the Changes Taking Place and the Agencies 



Involved. 



It is unfortunate that no synthesis of a soil has yet been effected, 

 and consequently the preceding analysis of the changes taking place 

 cannot be tested by reconstructing the whole process out of its con- 

 stituent parts. On the whole the evidence is satisfactory as to the 

 general course of the changes, but insufficient for sorting them out 

 quantitatively and precisely. The following scheme summarises them 

 as completely as is possible at present : 1 



^hydrates 



Waxes 



undcfomposed 



CO* C0 2 CaO>, C0 y 



* J 2 



FIG. 6. 



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

 by bacteria, 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 char- 

 acter. The fact that antiseptics put an end to most of the reactions 

 always used to be regarded as sufficient proof of their bacterial nature, 

 but this argument has lost much of its force since Bredig and others 2 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, 3 

 containing numerous colloidal bodies not much investigated, but con- 

 ceivably capable of acting as catalysts, and it is possible to imagine a 

 series of catalysts that would bring about all the known changes and 

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

 indeed been put forward from time to time : some ammonia is known 

 to be produced by chemical processes in the soil, Sestini (263) has 

 supposed that it is oxidised catalytically by the ferric oxide always 

 present to nitrites and nitrates, while Loew (184) states that nitrogen 

 can be catalytically " fixed " and converted into nitrates. Russell and 

 Smith (242) failed to reproduce these changes catalytically. Indeed the 



1 It will be noticed that these processes show certain resemblances to those of sewage 

 purification beds, as worked out by Adeney (i) and Fowler (98). For the decomposition of 

 fats, see Rahn (232). 



2 C/. Bredig and Ikeda, Zeit Physikal Chem., 1901, xxxvii., 1-68. 



3 See e.g. Joly and Smyth, Sci. Proc. Roy. Dublin Soc., 1911, xiii., 148-61. 



