THE TRANSFORMATION OF CARBON. 41 



Humus is the remains of life of previous generations. When 

 plants die, their roots, together with their leaves, branches, and 

 fruits, inevitably become incorporated into the soil. Animals, too, 

 leave upon the ground a quantity of excrement and other discharges; 

 and plants likewise probably discharge excretions into the soil. When 

 animals die their bodies also may become mixed with the earth. 

 Thus, practically all kinds of organic matter from animals and 

 plants are being mixed continually with mineral ingredients in the 

 surface layers of the soil. The microorganisms in the soil feed 

 upon these dead materials, causing an extensive series of decomposi- 

 tions and recombinations. To this mass of complex organic bodies 

 undergoing decomposition in the soil has been given the name 

 humus. It will be evident from this explanation of its origin that 

 humus cannot have a definite composition, and that it will hardly be 

 alike in any two soils. It will be composed of different materials 

 to start with, and there will be a variety of different stages of decom- 

 position. We cannot hope to find any definite composition of 

 humus, but we can study the kinds of decomposition and recombina- 

 tions that are going on in it and that result in making it a suitable 

 food for plants. In this study we must ever keep in mind the fact 

 that dead bodies of animals and plants are not in condition to serve 

 another generation of plants as food. We cannot feed plants upon 

 eggs, or urine, or starches, or sugars. Though containing carbon 

 and nitrogen in abundance, these elements are locked up in them out 

 of the reach of the green plants, and before they can be utilized 

 again they must be freed from their combinations and brought into 

 simpler forms. This is accomplished by the microorganisms in the 

 soil. Our study of these changes may best be centered around the 

 two chemical elements, carbon and nitrogen. 



THE TRANSFORMATION OF CARBON. 



The green plants seize the carbon dioxid (CO a ) from the air by 

 means of their leaves and, utilizing the energy of sunlight, build this 

 carbon into higher compounds. Starch is formed first, and later other 

 substances cellulose, wood, fats, sugar are built from the elements 



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