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 2 ) 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 
