ON MANURES. II31 



that are already present in the soil — those that are carried down to 

 the soil from the atmosphere in rain, dew, hoar-frost, and snow; 

 those that are applied artificially in fertilisers ; and those that are 

 formed in the soil from the nitrogen of other substances. 



As is well known, all the nitrogen that is applied to the soil 

 for fertilising purposes, especially in farmyard and stable manures, 

 compost heaps, in green - manuring, and by animal manures, is 

 not in the form of nitrates. It may be either in the form of 

 ammonia or of more complex organic compounds. It is very 

 probable, however, that before it is taken up by the growing 

 plant the organic nitrogen is changed, first into the form of 

 ammonia, and then into nitric acid. 



These changes all take place through the agency of micro- 

 organisms, or ferments, in the soil, and that particular process in 

 which the nitrogen of the ammonia is changed into nitric acid 

 is called nitrification. This change is accomplished by the joint 

 action of two separate organisms, one of which changes the 

 nitrogen of ammonia into nitrous acid, while the other changes 

 the nitrous acid into nitric acid, the latter being the form in 

 which it is assimilated by plants. 



The conditions that are required for the development of nitri- 

 fying organisms are the presence in the soil of certain food- 

 constituents — heat, moisture, oxygen, and some mineral base — to 

 neutralise the nitric acid as it is formed. It is also necessary that 

 the soil be slightly alkaline. The nitrifying organisms require 

 certain substances as food, among which phosphoric acid and 

 lime are most important. It has been found that without phos- 

 phoric acid there can be no nitrification. This probably is one 

 of the reasons why phosphatic manures show such beneficial results 

 when applied to certain soils, as well as furnishing direct plant- 

 food. 



The three conditions which exert a marked influence on 

 nitrification, and which in horticultural practice are more or less 

 intimately associated, are heat, air, and moisture. The process 

 is most rapid during warm weather, in presence of sufficient air 

 and moisture. Hence it is more active in summer than in 

 winter, and more rapid in a conservatory or covered frame than 

 in the open garden. 



Nitrification teaches the gardener, also, the reason why 

 thorough tillage of the soil is so essential, and why charcoal, 

 added to potting material and a sufficiency of drainage crocks, 

 in pot-culture, is so important and beneficial. The loosening 

 and pulverising of the soil allows the admission of necessary 

 oxygen and regulates the supply of moisture. If the soil is 

 allowed to become very dry, or, on the other hand, is saturated 

 with water to the exclusion of air, nitrification is retarded, and 

 may be permanently stopped. In this connection it is interesting 

 to note that in certain plant-cultures, especially that of the 



