THE PROPERTIES OF SOILS 207 



show any marked reaction for nitrates ; in the other a few moulds 

 will be growing, and nitrates may be found if the added soil had 

 contained much calcium carbonate. The soil contains bacteria 

 which slowly change other compounds of nitrogen like ammonia 

 into nitrates, but which can only work in a solution kept neutral 

 by the presence of a base like carbonate of lime. 



If the liquid in flask 6 be also examined when two or three 

 months have elapsed it will show a reaction for nitrates, although 

 no nitrogen compound had originally been added to the flask. 

 The nitrogen it contains has been gathered from the atmosphere 

 by the bacteria forming the brown skin first appearing, the trans- 

 formation of which into nitrate is the later work of another group 

 of bacteria. 



A good example of the cycle of changes through which nitro- 

 genous materials in the soil are always passing may be obtained 

 by making up another flask of nutrient solution as previously 

 described, and adding to it both the half gram of calcium carbonate 

 and 0-2 of a gram of peptone (one of the most complex nitrogenous 

 compounds), inoculating with soil as before. At the end of a 

 week or so evidence of the first change to take place putrefaction 

 will be obtained from the smell of the flask, while the liquid 

 will be seen to be turbid with the numbers of putrefactive organ- 

 isms that have developed. After another week or fortnight 

 the putrefactive smell will be exchanged for a faint smell of 

 ammonia, the formation of which represents a still further step 

 in the break down of the peptone. Again, leave the flask in 

 the dark for a month and its contents will begin to show the 

 reaction for nitrates, which are now being produced from the 

 ammonia. Now bring the flask into the light for some weeks ; 

 in time a green growth of algae will appear, accompanied by 

 the evolution of oxygen when the light is bright. At the same 

 time both nitrates and ammonia will have disappeared from 

 the liquid ; they have been reconverted into protein in the sub- 

 stance of the algae. Thus after the bacteria have taken the 

 original protein and reduced it step by step to nitrates the plant 

 steps in, and by the aid of the energy of light rebuilds another 

 complex protein. 



It is impossible at this stage to discuss with any particularity 



