FERTILITY IN SOILS 355 



parently the equilibrium does not vary much so long as the 

 soil conditions remain fairly constant. 



The general outline of the changes, so far as they have been 

 traced out, is somewhat as follows. The organic matter is finally 

 resolved into carbon dioxide, water, nitrogen, ammonia, calcium 

 carbonate and other mineral matter, as well as into more or less 

 stable organic substances which tend to accumulate in the soil. 

 Decomposition takes place slowly, however, a number of inter- 

 mediate products of the humus type, of varying degrees of 

 stability, being found ; the low temperature and rather special 

 conditions apparently favour unstable molecular groupings. 

 The most remarkable feature of the decomposition and the one 

 most in need of further elucidation is the evolution of consider- 

 able quantities of gaseous nitrogen. 



Normally ammonia does not remain as such in the soil but 

 is either absorbed by some of the clay constituents to form a 

 curious compound not yet investigated, or it is oxidised by 

 bacteria to nitrite, and finally to nitrate. The nitrifying 

 organisms derive their carbon not from organic matter, which 

 indeed is rather injurious to them, but from carbonic acid, 

 which they assimilate and convert into complex cell substances 

 without the aid of sunlight or the intervention of chlorophyll ; 

 apparently they utilise the energy set free by the oxidation of 

 ammonia. 



Another set of organisms possesses the remarkable property 

 of absorbing gaseous nitrogen from the air and converting it 

 into protein. A considerable amount of energy is of course 

 necessary and is derived from the oxidation of organic matter. 

 Between this fixation of nitrogen and the liberation of nitrogen 

 already mentioned there is generally an equilibrium. The steps 

 in these changes are entirely unknown. 



There are three general methods of studying the effects 

 of these changes on soil fertility. The separate changes may 

 be traced out ; the effect of the whole series of changes 

 may be estimated ; the micro-organic flora may be altered and 

 the results observed. All are indirect ; unfortunately no direct 

 means of studying the separate bacterial processes at work in 

 the soil is yet available. It is impossible at present either to 

 make soil artificially or to reconstruct the bacterial flora in a 

 sterilised soil ; it is even impossible to sterilise soil without 

 profoundly altering its character. 



