VII.] BACTERIAL ACTIONS IN SOIL 145 



assumes will depend very greatly on external conditions, 

 such as the cultivation the land receives. Perhaps the 

 most important are the organisms concerned with 

 preparing the food for plants ; in the first place the 

 putrefactive organisms break down proteins and kindred 

 nitrogenous material into successively simpler com- 

 pounds, which are then taken in hand by another group 

 of bacteria and converted into ammonia, though it is 

 impossible to draw any hard and fast distinction 

 between these two sets of organisms. The work of 

 the ammonia - makers is concluded by the nitrifying 

 organisms, which complete the final stage of oxidation 

 of the nitrogen compounds and produce the nitrates 

 forming the chief source of nitrogen for our crop plants. 

 The nitrifying organisms are on the whole so much 

 more active than the ammonia-makers that there is 

 rarely more than a trace of ammonia to be extracted 

 from the soil ; the rate at which nitrates are ready for 

 the plant is mostly determined by the rate at which the 

 ammonia-makers can produce ammonia to be nitrified. 

 The work of all this group of organisms is promoted by 

 the same factors as forward the growth of other living 

 creatures, i.e.^ a proper degree of warmth and moisture 

 and a due amount of soluble food (in this case the 

 mineral salts found in plant ashes). It is also 

 stimulated by such special conditions as an abundant 

 supply of air, a sufficiency of carbonate of lime, and 

 cultivation to distribute the bacteria in the soil. On the 

 other hand, under certain conditions, as when sulphate 

 of ammonia or nitrate of soda is applied to the soil, 

 many organisms will seize upon these soluble nitrogen 

 compounds for their own nutrition and multiply in such 

 numbers that an appreciable fraction of the nitrogen in 

 the fertiliser is withdrawn in order to build up the 

 bodies of the bacteria, which becoming thereby con- 



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