THE SOIL 



325 



The organisms in the soil are very various and do dif- 

 ferent kinds of work. Some take up the work where others 

 leave off, and carry it a stage farther. Much oxygen is 

 used up in the process, and in consequence the air in the 

 soil is poorer in oxygen and richer in carbon dioxide than 

 the atmosphere above ; however, the supply of oxygen is 

 kept up by diffusion. 



In addition to bacteria and moulds, other and bigger 

 organisms are at work, such as earthworms, millipedes, 

 centipedes, beetles, and the larvae of many insects, all acting 

 on the soil in complex ways. For example, the burrows of 

 earthworms serve as air- and water-channels and make it 

 easier for roots to penetrate ; while the worm-castings 

 contribute much to the fine dark soil on the surface, and 

 render it more soluble. 



Nitro-bacteria and the origin of nitrates. The process 

 called nitrification is an excellent illustration of the work 

 of soil-organisms. During the changes which take place in 

 the organic matter in the soil, owing to the action of one 

 set of bacteria, nitrogen and ammonia are formed. The 

 nitrogen escapes into the air and the ammonia is acted upon 

 by another bacterium and converted into nitrous acid. 

 This acid readily reacts with certain mineral substances in 

 the soil (e. g. carbonates) to form salts called nitrites. A 

 third group of organisms converts the nitrous acid into 

 nitric acid ; and the action of the free nitric acid on salts 

 of calcium, potassium, and sodium gives rise to compounds 

 like potassium nitrate (saltpetre), calcium nitrate (lime- 

 saltpetre) , and sodium nitrate (Chili saltpetre) . These com- 

 pounds are of great value to plants ; they are very soluble in 

 water, and therefore can be absorbed by the root-hairs ; 

 they are the only source from which most flowering plants 

 obtain their nitrogen. 



The organisms which bring about these important changes 

 are called nitro-bacteria, and they differ from other organisms 



