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The exact manner in which micro-organisms reduce the nitrogenous 

 stores of humus to the form of ammonia is, of course, not known, and the 

 ferments which are active therein have been the subject of less investi- 

 gation and are more imperfectly understood than those which are active 

 in the formation of nitrous and nitric acids. 



It may be possible that the organism which converts organic matter 

 into carbonate of ammonia, and that one which forms nitrous acid, are 

 quite similar in character, but this cannot be definitely stated. 



PRODUCTION OF NITROUS ACID. 



The next step in the process of nitrification is the conversion of am- 

 monia or its compounds into nitrous acid. With a moderate store of 

 ammonia, the oxidation into nitric acid takes place, as a rule without 

 any of the nitrogen being lost in a free state or being volatilised as am- 

 monia compounds. When, however, there is a large excess of am- 

 monium carbonate, a considerable loss of nitrogen may take place. The 

 practical deduction to be drawn from this fact is apparent. Nitro- 

 genous fertilisers should be applied only in moderate quantities, so as 

 not to increase the stock of material beyond the power of the active fer- 

 ments to handle it. 



The nitrous ferment is by far the largest and most vigorous of the nitri- 

 fying organisms. It is from three to four times as large as the nitric fer- 

 ment, and under a higher power of the microscope appears as minute 

 globules, slightly oblate. These globules are multiplied by spores which 

 develope rapidly to perfect organisms of full size. In most cases the 

 organisms appear as distinct globules, but many are congregated into 

 masses where the distinctive cell structure seems to be lost. 



CONVERSION OF NITROUS INTO NITRIC ACID. 



The last step in the process of nitrification consists in the oxidation of 

 nitrous to nitric acid. As a rule plants absorb nitrogenous food only as 

 nitric acid, but it cannot be said that the nitrogen may not be used by the 

 plant in other forms. Some experiments seem to show that ammonia 

 and its compounds may be directly absorbed by plants, but if this be 

 true it must be only in a very limited quantity. The final step, therefore, 

 in nitrification is necessary to secure this valuable food in its most highly 

 available state. The nitrifying organisms are much smaller than their 

 nitrous cousins, and of the same general shape but more globular. 



It must not be supposed that these steps in the preparation of a nitro- 

 genous food are performed with entire distinctness. The impression might 

 be obtained that the ammoniacal ferment exerted its activity, converting 

 the whole of the nitrogenous supply into ammonia, and that in this state 

 only the nitrous ferment would become active and convert the whole 

 product into nitrous acid which finally, under the influence of the nitric 

 ferment, would form nitric acid. In point of fact, however, in arable soils 

 and under favourable conditions the steps of nitrification may be almost 

 synchronous. In the case of a growing crop, a chemical examiuation 

 may find only traces of ammonia and nitrous and nitric acids. As each 

 particle of ammonia is formed it is converted without delay into nitrous 

 acid, and then at once into nitric acid. The nitric acid formed would be 

 absorbed by the growing plant, and thus it might seem that the activity 

 of the ferments present in the soil had been reduced to a minimum, when 



