90 SOIL CONDITIONS AND PLANT GROWTH 



In these experiments mixed cultures were used, the nitrate producers 

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

 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 vigor- 

 ously in both cases. An exception was therefore made in favour of 

 " humus " (208). 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 sufficient excess 

 of calcium carbonate is therefore necessary both in culture solutions 

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

 tions the nitrate producer is somewhat sensitive even to ammonium 

 salts, indeed both Warington (296) and Omelianski (p. 89) suppressed 

 it by maintaining a sufficient concentration of ammonium sulphate ; 

 Lohnis has shown, however (185), that it is more tolerant in the soil. 

 Some substance toxic to them is produced when soil is heated to 98 C. 

 or more, and in such soils they cease to act. Neither nitrosomonas nor 

 nitrobacter has been observed to form spores, or to survive temperatures 

 above 45 C., or treatment with mild antiseptics like carbon disulphide 

 and toluene. But so widely distributed are they and so readily can they 

 spread in the soil, if the conditions are at all favourable, that they may 

 reappear unless special precautions -are taken to prevent infection. 

 Thus, it is commonly stated that treatment of the soil with carbon 

 disulphide merely depresses without killing the organisms. Russell 

 and Hutchinson found, however, that the organisms did not reappear if 

 the soil was kept carefully free from re-infection (240^). 



