THE AMERICAN BOTANIST. 57 



Nitrification of Soils — Although plants are able to 

 take up the ammonia dissolved in soil water, they seldom 

 do so under normal conditions. The principal reason for 

 this is that a host of organisms in the soil are constantly 

 turning the ammonia into a more readily assimilated 

 forni of plant food. These organisms are known as bac- 

 teria and two different kinds are required to complete the 

 process. The first bj' combining the ammonia and chalk 

 or lime in the soil with the oxygen of the air, forms as 

 products calcium nitrite, carbon dioxide and water. The 

 organisms that perform this part of the work belong to a 

 group called Nitrosococcus. At this juncture a new set of 

 organisms called Nitrobacter, about one tenth the size of 

 the others, take up the nitrite of lime and by the addition 

 of more oxygen turn it into a nitrate of lime which the 

 plant readily uses. Until we know why these organisms 

 work so steadily at nitrification the3^ seem the most 

 philanthropic of bacteria. It turns out, however, as one 

 might expect, that they are doing this work without refer- 

 ence to any but themselves. The higher plants absorb 

 carbon dioxide from the atmosphere, and splitting it up 

 in the interior of the leaf, retain the carbon and give off 

 the ox^'gen. The energy necessary to do this is obtained 

 from sunlight by the green coloring matter in the plant. 

 The niti'ifying bacteria also need carbon dioxide, but as 

 they have no green coloring matter, can not make use of 

 the energy from the sun's rays in splitting thisgas into its 

 component elements and in fact the process of nitrification 

 goes on only in the dark. To obtain their energ}-, then, 

 these organisms have resorted to oxidizing ammonia and 

 using the energ\^set tree in the process. Experiments have 

 shown that these organisms are able to assimilate one 

 part of carbon for every 42 parts of ammonia oxidized to 

 nitrite. Nitrification proceeds most rapidU' in a warm 

 moist soil. The important part that lime and chalk play 

 in this process is significant and may throw new light up- 

 on the liking which certain plants, especially ferns, have 

 for a limestone soil. 



