NOURISHMENT OF PLANTS. 41 



matter decomposing in the soil is in a condition to 

 enable it to convert a part of the nitrogen absorbed 

 from the air directly into ammonia ; a circumstance 

 which of course renders the presence and retention 

 of humus in arable land of great importance. 



4. The inorganic or mineral substances requisite to 

 the growth of plants are conducted into their struc- 

 ture through the soil and the water. The original 

 mass of our various soils consists of crumbled rocks, 

 and these for the most part, except perhaps fossil 

 silica or pure bog-earth, contain all the mineral sub- 

 stances which plants require for their support, al- 

 though some of them in very inconsiderable quan- 

 tity. In the solid rock these ingredients are insol- 

 uble in water; but nature provides for this, inas- 

 much as from year to year some portion of its mass 

 is loosened and decomposed. This is accomplished 

 by the process of weatherings which plays the same 

 part in the preparation of the inorganic nutri- 

 ents of plants, that putrefaction and decay perform 

 in the elaboration of those which are organic. 

 Chemical forces here, assisted by air and water, 

 warmth and cold, plants and animals, effect at 

 length the formation of pulverulent soils from solid 

 rocks and the generation of soluble salts from insolu- 

 ble mineral combinations, which salts may in that 

 state be taken up by the roots of plants. 



But weathering takes place also below the sur- 

 face of the earth, and indeed wherever air and wa- 

 ter can penetrate into the mass of rock. The sub- 

 4* 



