160 BE VIEW OF THE PRINCIPLES OP NUTRITION. 



'? 



water and from the carbonic acid of the atmosphere ; hydrogen fron 

 water and ammonia, and nitrogen from ammonia and nitric acid, drawi 

 either from the atmosphere or the soil. 



836. THE ATMOSPHERE CONTAINS about ar^ part of carbonic acid, diffused through 

 out its whole extent ; and as this gas contains 27 per cent, of carbon, it is demon 

 strable that the whole atmosphere contains morejjjan 600 billions (600,000,000,000 

 of tons of solid carbon, derived from the sources already mentioned ( 835), 

 an amount fully adequate to the demands of the vegetable kingdom. 



837. SOIL consists of two classes of materials, viz. : mineral, and organic. Th 

 former, called earths, consists of disintegrated and pulverized rocks, including a] 

 the various mineral substances which are found to enter into the composition ol 

 plants, as potassa, sod^ silica, lime, etc., all of which are more or less soluble ii 

 water. The organic materials consist of the remains of former tribes of plants am 

 animals mingled with the earths ; and which, having access to the air, are decom 

 posed, evolving carbonic acid and ammonia both to the air and the water. 



838. WATER is composed of oxygen and hydrogen (HO) hi the proportion of 

 to 1 by weight, or one atom of each to each. Having pervaded the atmosphere ii 

 the state of vapor and rain, and percolated through the soil, it holds in solu 

 tion carbonic and nitric acids, ammonia, and many of the various minerals abov 

 mentioned. 



839. AMMONIA consists of nitrogen and hydrogen combined in the proportion o 

 one atom of the former to three of the latter (N H 3 ). It arises from decaying ani 

 mal and vegetable matter, as above stated, and is diffused through the atmospher 

 in the proportion of about 1 part in 10,000. 



840. NITRIC ACID is ateo believed to yield nitrogen to plants. It consists of on 

 atom of nitrogen to five of oxygen (N 3 ). During thunder-storms it is generate' 

 in the air by lightning and brought down by rain. When combined with th 

 bases, as potassa, soda, etc., it forms nitrates substances known to be efficient fei 

 tilizers in soils. 



841. AIR PLANTS. Thus it appears that water, carbonic acid and ammonia (o 

 nitric acid) may yield to plants their four essential organic elements. And since a] 

 of them are contained in the atmosphere, some plants are capable of subsisting 01 

 air alone (long moss, lichens) ; but most species are dependent on water, earth, am 

 air, and demand a copious supply. 



842. THE EXTERNAL CIRCUMSTANCES, therefore, first requisite t< 

 healthy vegetation are, 1, free access to an atmosphere which is oftei 

 agitated by winds ; 2, a proper supply of rain or river water ; 3, a soi 

 possessing the peculiar minerals required by the species to be growi 

 upon it, together with a certain proportion of vegetable mold. 



843. THE SUPPLY. The first of these is everywhere abunclantb 

 supplied by nature, and asks no aid from man. The second and thir< 

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

 By irrigation streams of water are turned from their natural channels t< 

 add to the scanty moisture of fields parched with drought ; by drain 

 age the inundated bog is converted into a luxuriant lawn. 



844. THE OBJECT OP 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 decompo 

 sition. Subsoiling, or deep ploughing, is an operation whereby that stratum of eartl 



