88 



SOIL CONDITIONS AND PLANT GROWTH 



dioxide is an endothermic process requiring a supply of energy. In 

 the case of the green plant, the only other living thing known to 

 utilise carbon dioxide, the eneigy comes from light, the transformer 

 being chlorophyll. Here, however, light is out of the question, and 

 is even fatal to the organism. Winogradsky (311) suggested that the 

 necessary energy is afforded by the oxidation of ammonia and of the 

 nitrite, and he traced a definite relationship between the amount of 

 ammonia oxidised and the carbon assimilated : — 



In these experiments mixed cultures were used, the nitrate producers 

 predominating. More recently Coleman (71), using pure cultures or 

 nitrate producers, obtained ratios varying from 40 to 44. 



No useful hypothesis has yet been put forward to account for these 

 remarkable facts. The whole subject deserves serious attention from 

 some competent chemist. 



It was somewhat hastily inferred that organic matter would have a 

 retarding effect in the soil just as it has in culture solutions. From 

 the outset, however, certain facts were known to be against this view : 

 thus, there was a good deal of organic matter in the old nitre beds (235) 

 and also in rich gardens, and yet nitrification went on vigorously in both 

 cases. An exception was therefore made in favour of " humus " (208 

 and 209). Later on Adeney (i), and again Miss Chick (70), found 

 another exception : the organic matter of the filter beds used in sewage 

 purification. Coleman has now shown (71), and Stevens and Withers 

 (272) have confirmed it, that only in culture solutions is organic matter 

 injurious : in the soil it does no harm, and may even help the process. 

 Thus quantities of dextrose that stopped nitrification entirely in Win- 

 ogradsky and Omelianski's culture solutions were found to act bene- 

 ficially in soil under normal conditions of temperature and moisture 

 content. The discrepancy cannot yet be explained. Sucrose, lactose, 

 and certain other non-nitrogenous compounds had no effect, but nitro- 

 genous compounds were distinctly injurious. 



The organisms will not tolerate an acid medium ; a sufiScient excess 

 of calcium carbonate is therefore necessary both in culture solutions 

 and in soils. Nor will they tolerate free ammonia. In culture solu- 



