THE SOIL AND ITS IMPROVEMENT. 95 



know to-day. The lightning would cause a little of the elements 

 of the air to unite, forming a substance called nitric acid, which 

 the rains washed out and brought down to the slowly forming 

 soil, thus adding the substance, nitrogen. 



Low forms of vegetable life were placed upon the earth. 

 Mosses and lichens clung to the rocks and corroded them, draw- 

 ing from them material needed for their own life. When these 

 died, the substances thus obtained were added to the soil. 

 Higher forms of life were placed upon the soil, taking from 

 it nitrogen, phosphoric acid, potash, and some other substances 

 we need not mention here, and gathering from the air the sub- 

 stance, carbon, which we see so often in the form of charcoal. 

 This exists in the air in the form of an invisible gas, but the 

 plant can take it from the air and change it into a solid form. 

 When these plants died and decayed they added this new sub- 

 stance, carbon, to the soil, and it began to assume a dark color. 



This work went on through the years and ages, and very 

 likely through thousands of ages, and the soil was constantly 

 being increased in quantity by the destruction of the rocks, 

 gaining from them potash, phosphoric acid, and other mineral 

 elements of plant life, and gaining from the rains nitrogen, in 

 the form of nitric acid. The growing plants took these from 

 the soil, added the carbon from the air, worked it all up into 

 forms of life, and, dying, returned to the soil all that had been 

 taken from it, and the carbon from the air besides. This car- 

 bon, thus added to the soil, could not be taken up by the roots 

 of plants, but it made a suitable bed for them to grow in, and 

 formed a soil that could retain the warmth and moisture so 

 needful to their growth, and which was porous, to admit the 

 air, also needed. As the process of decay in the soil continued 

 further, this carbon gradually united with the oxygen of the air, 

 forming carbonic acid, to begin the great circuit over again. 

 Thus excess of this substance in the soil was avoided, except in 

 places where the soil was always soaked with water. In these 

 places the carbon of the decaying plants could not unite with 

 oxygen from the air, and it accumulated, forming beds of muck, 

 or peat, composed principally of carbon. 



