THE ORIGIN OF SOIL. 65 



ORGANIC >r HSTANCES. 



The fertility of any soil depends chiefly upon the organic 

 substances present. It is these that account for most of the 

 difference between the fertility of sand and garden soil. A 

 sandy soil may contain all the necessary mineral ingredients 

 for plant life, but its power of supporting vegetation is very 

 limited, while a garden soil, having, in addition to the minerals, 

 a goodly quantity of organic bodies, is very fertile and supports 

 plant life abundantly. 



Some of these organic bodies in the soil are easily enough 

 understood. It is evident that fertile soil contains a consid- 

 erable amount of dead and decaying animals and plants 

 which contribute to its composition. But, in addition to the 

 apparent decaying substances, the soil contains a considerable 

 quantity of a material called humus. This is an ingredient 

 rather difficult to define. It is abundant in fertile soil and 

 scarce, or wanting, in barren soil. It represents the final con- 

 dition into which the decayed animal and vegetable tissues have 

 been brought by the series of changes they have undergone in 

 the soil. 



The humus is made up of many different chemical bodies, 

 chiefly composed of carbon, hydrogen, oxygen and nitrogen, 

 although containing other elements in some quantity. The 

 most significant of these elements are carbon and nitrogen. 

 In our study of plant food the problem of the oxygen and hy- 

 drogen need not detain us, since the plants can assimilate these 

 two elements directly from the air and from the water of the 

 atmosphere, and are therefore able to obtain an unlimited 

 quantity. Carbon is the most characteristic element of or- 

 ganic bodies. This the plants obtain from the atmosphere in 

 the form of CO 2 , a gas which is present in small proportions, 

 the air containing less than o. I per cent., and the problem of 

 how this supply is maintained is one of some interest. But 

 more important is the question of the transformations of ni- 

 6 



