SAPROPHYTES 403 



and again combine it in a form which serves as food. Thus it 

 is that bacteria, and other agents, such as other non-green 

 plants and animals keep constantly returning carbon dioxide 

 to the air where it becomes available for the green plants. 



459. Soil Bacteria and Nitrogen. We have seen in Chap. 

 VII, Art. 377, that only a comparatively small number of 

 chemical elements are needed to make up the numerous 

 chemical compounds that constitute the human body. This is 

 true of all living bodies. Among these few necessary elements, 

 there is none that gives us greater concern than nitrogen. 

 This element constitutes approximately four-fifths of the 

 atmosphere, but it is not found in a combined form in the 

 natural rock of the earth. It is therefore not in the soil in a 

 combined state except as it has been put there by living organisms. 1 

 On this account, the nitrogen supply of the soil is usually more 

 limited than that of the other soil elements which were present 

 in the original rock of the earth from which the soil was made. 

 At the same time no living thing can exist without nitrogen 

 for it is one of the necessary elements in the makeup of 

 protoplasm. 



The green plants take up the nitrogen from the soil in certain 

 definite chemical compounds, usually in the form of NITRATES, 

 such as potassium nitrate. The green plant has the power of 

 combining the nitrogen in this form with other elements to 

 form proteins and these are food for the green plant and for 

 other organisms. The nitrogen which is thus formed into 

 protein compounds by green plants is practically the sole 

 available supply of nitrogen for the animal world, for animals 

 do not have the power of making proteins out of the nitrates 

 or other similar compounds. We get proteins when we eat 

 the flesh of other animals, but this, in turn, may ultimately 

 be traced back to proteins formed by some green plant (see 

 Art. 380). 



1 NOTE : Small amounts of combined nitrogen are carried down by rain during 

 storms, especially thundei showers. Lightning causes some of the nitrogen 

 and oxygen of the atmosphere to unite into chemical compounds, gases, which 

 are absorbed by the falling rain. 



