ORGANIC ELEMENTS OP SOIL. 55 



be drawn from this, is the sixth principle of agricul- 

 tural chemistry, that soil, consisting chiefly op 



ONE SILICATE, OR SALT, IS ALWAYS BARREN. 



92. It is not probable that soil, thus chemically 

 constituted, exists. Admitting such to occur, even 

 then, when dressed with the food of plants, it 

 would not be fertile. The want of a mixture of 

 earthy ingredients, which are as essential to the 

 growth of plants, as are air and moisture, would ef- 

 fectually prevent the growth of crops. Only a por- 

 tion of the elements, thus essential to plants, exists 

 in them, in that state, in which they exist in soil. 

 The silica, and potash, and lime, exist in plants as 

 in soil, as silicate of potash, and sulphates and phos- 

 phates of lime and potash. When the ashes of 

 plants are examined, we find carbonates of bases, 

 which did not exist as such in the soil. A large 

 portion of carbonates of lime and potash is found in 

 ashes. 



93. The origin of these, is to be sought in acids, 

 which, by heat produce carbonic acid. This is the 

 effect of heat upon all salts, formed of vegetable 

 acids. These are tartaric, malic, citric, and acetic 

 acids. The inorganic elements of plants, exist in 

 combination chiefly with organic or vegetable acids. 

 Each plant forms acids, in definite quantity, propor- 



