78 The Substratum 



after it percolates through the ground as soil water. But the action of 

 pure rain water would be extremely slow. Root secretions added to 

 ground water, including notably carbonic acid, cause the rock mate- 

 rials to go into solution much more rapidly. If you look closely at a 

 rock covered by lichens, you can see the results of the combined action 

 of mechanical breakdown and corrosion due to plant secretions. 



The third factor taking part in the process of soil formation is the 

 addition of organic matter. Plants contribute their deciduous parts 

 at regular intervals, and when each plant dies it adds its whole 

 body to the soil. Since one thinks of plants as growing by withdraw- 

 ing material from the soil, one might ask how any net gain would 

 result from the death of the plant and the return of this material to 

 the substratum. The answer is that the plant builds additional 

 materials from the air and the ground water into its tissues. The 

 carbohydrate synthesized by the plant is formed from carbon dioxide 

 and water absorbed from the environment. Certain bacteria, such 

 as Azotobacter and Clostridium, living freely in the soil, are able to 

 take nitrogen from the air and to use it in their constructive growth. 

 Other nitrogen-fixing bacteria live in nodules on the roots of legu- 

 minous plants to which they pass on the nitrogenous compounds that 

 they have manufactured. Thus, more is added to the soil by the 

 activity of the vegetation than is removed, and, what is more im- 

 portant, the new material is in a very different chemical form. The 

 inorganic substances taken up from the soil and from the atmosphere 

 are converted into complex organic compounds by the growth proc- 

 esses of the plant. When the plant dies, these organic substances 

 become incorporated into the soil and many of them decompose only 

 slowly. 



Animals living on and in the soil add their excreta regularly and 

 contribute their own bodies to the substratum when they die. Bur- 

 rowing animals of all sorts mix into the soil the organic remains that 

 have been added to the top of the ground. Rodents and many kinds 

 of insects play an important role in this regard, as well as the pro- 

 verbial earthworm. A visit to a deciduous forest where earthworms 

 are abundant will provide an opportunity for seeing the effectiveness 

 of this animal in tilling the soil. If you remove from the surface of 

 the ground the leaves that accumulated in the last few months, you 

 find few leaf remnants from previous years. All the older leaves 

 have been eaten by the earthworms, and their faeces have largely been 

 discharged at subsurface levels. When constructing new burrows, 

 earthworms deposit their casts of soil from the deeper levels upon the 

 surface. This action of the earthworms in mixing the upper layers in 



