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sufficient for the formation of all the banks of coral in the ocean, and 

 the various shells of the marine animals. 



Having given this account of the constituents of plants, and the sourc- 

 es whence they are derived, Liebig proceeds to discuss the art of cul- 

 ture and the action of manures. I should be glad to quote the whole of 

 this chapter, but must limit myself to a brief sketch. Humus is not sol- 

 uble in water; if it were, a great part of it would be carried off the 

 ground by rains. Its office is, by the presence of water to convert the 

 surrounding oxygen into carbonic acid, which plants absorb, and then re- 

 turn to the soil a large portion of carbonaceous matter, that they abstract 

 from the air, so that the humus of the soil is not diminished. The fre- 

 quent ploughing of the soil, so as to promote, by admitting the oxygen 

 of the air to the humus, the formation of carbonic acid, the application 

 of alkaline substances, and whatever tends, as Liebig expresses it, to 

 put the organic matters of the soil in a state of oxidation, increase the 

 fertility of the soil. The oxygen then assists in the formation of car- 

 bonic acid to go to the nourishment of plants. 



Knowing the substances which go to form the plants, the object of a 

 wise agriculture will be to supply them, and to render them accessible 

 to the plants. Potash, the most common and important of the inorganic 

 constituents of plants, is more universally and abundantly diffused over 

 the earth than any other alkaline substance. But the alkalies, by con- 

 tinual cultivation, may be exhausted, and the soil cease to be produc- 

 tive. This indicates the necessity of a fallow or rest to the soil, by 

 which, under the operation of air aud moisture, a further disintegration 

 of the rocks may take place so as to furnish the necessary alakalies to 

 the soil, or, without resting, they may be artificially supplied. Plants 

 themselves in their decay return alkaline substances to the earth ; and 

 it is well ascertained that plants themselves act powerfully in the disin- 

 tegration of rocks. 



Some crops may be repeated on the same soil more frequently than 

 others, because some consume more of the alkalies than others. One 

 hundred parts of the stalks of wheat yield 15.5 parts of ashes. The 

 same quantity of barley, 8.54 parts ; and of oats, only 4.42. The 

 ashes of these different plants are of the same description, but it is ob- 

 vious that the demands which they make upon the soil must be different. 



The interchange of rotation crops and the application of manures 

 are materially connected with this fact, and with another in the habits 

 of plants, to which we shall refer. Plants of different kinds absorb or 

 take up different substances, from the soil ; and^one kind therefore may 



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