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 peculiar 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 ¢trrigation, streams of water are turned from their 
natural channels to add to the scanty moisture of fields parched 
with drought; while, by drammage, the inundated bog is con- 
verted into a luxuriant lawn. 
320. The object of tillage is to pulverize and kghten 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- 
img is mainly to increase the quantity of organic matter. By 
various amendments, 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 which contain it, 
as granite, clay-slate, basalt, &c. 
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