108 REVIEW OF THE PRINCIPLES OF NUTRITION. 



in the state of vapor and rain, and percolated through the soil, 

 it holds in solution carbonic acid, ammonia, and many of the 

 various minerals above mentioned. 



317. Ammonia consists of nitrogen and hydrogen, in the pro- 

 portions of 14 to 3 by weight. It arises from decaying animal 

 and vegetable matter, as above stated, and is also generated in 

 the atmosphere, during storms, by the flashes of the electric 

 fluid. 



318. Thus it appears that the three compounds, water, car- 

 bonic acid, and ammonia, may yield to plants their four essential 

 organic elements. And, since all of them are contained in the 

 air, some plants are capable of subsisting on air alone ; but most 

 species are dependent on water, earth, and air, and demand a 

 copious supply. The external circumstances, therefore, first 

 requisite to healthy vegetation are, — 



1. Free access to an atmosphere which is often agitated by- 

 winds. 



2. A proper supply of rain or river-water. 



3. A soil possessing the pecuhar minerals required by the 

 species to be grown upon it, together with a certain proportion 

 of vegetable mould. 



319. The first of these is everywhere abundantly supplied by 

 nature, and asks no aid from man. The second and third are 

 often deficient, and are to be supplied by the labors of agricul- 

 ture. By irrigation, streams of water are turned from their 

 natural channels to add to the scanty moisture of fields parched 

 with drought ; while, by drainage, the inundated bog is con- 

 verted into a luxuriant lawn. 



320. The object o'l tillage is to pulverize and lighten the too 

 compact soil, and thus expose every part to the oxygen of the 

 air in order to hasten its decomposition. The object of manur- 

 ing is mainly to increase the quantity of organic matter. By 

 various atnendmcnts, as gypsum, lime, and pulverized charcoal, 

 ammonia is powerfully attracted from the air, and yielded again 

 to the water. Marl promotes the decomposition of the soil, and 

 ashes add to the potassa which exists naturally in it being 

 derived from the decomposition of the rocks wliich contain it, 

 as fjranite, clay-slate, basalt, &;c. 



