OF PLANTS BY MANURING. 65 



bestow especial consideration upon this substance in 

 manuring his land ; inasmuch as it forms the prin- 

 cipal mass of our solid earth, is absent in no soil 

 (with the exception, perhaps, of pure humus or bog- 

 earth), and is, moreover, contained in all spring and 

 river water. His single care must be directed to the 

 labor of insuring that the silica be rendered soluble^ 

 in order that it may be able to penetrate into his 

 plants. He incidentally attains this end by deep 

 ploughing, good manuring, or the application of 

 lime, since in this way he affords to the soil ammo- 

 nia, potash, lime, etc., which are solvents of silica. 

 In most cases, however, the farmer is exonerated 

 even from this care ; for nature, which, by weather- 

 ing, liquefies from year to year a portion of this 

 earth, takes this labor off his hands. The solid 

 excrements of animals are particularly rich in si- 

 lica. 



The same observation applies also to the remain- 

 ing constituents of manure, — oxide of iron^ sulphu- 

 ric and muriatic acids^ with other substances which 

 have not yet been specified. They are so generally 

 diffused in the earth and in water, that plants prob- 

 ably find in every soil as much of them as may be 

 needed for their growth. Thus, for example, gyp- 

 sum consists of lime and sulphuric acid ; common 

 salt, of soda and muriatic acid ; but small quantities 

 of gypsum and common salt are present in almost 

 every kind of earth, and in all water. 



Of the ingredients of manure which have hither- 

 6* 



