ORGANIC ELEMENTS OF SOIL. 67 



stant in plants. The deduction to be drawn from this is, 

 the sixth principle of agricultural chemistry, soil, consisting 



CHIEFLY OF 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 effect- 

 ually prevent the growth of crops. Only a portion 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 phosphates 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. Such are tartaric, 

 malic, citric, oxalic, and acetic acids. The inorganic ele- 

 ments of plants exist in combination chiefly with organic or 

 vegetable acids. Each plant forms acids, in definite quan- 

 tity, proportionate to the size, age, and part of the plants ; 

 the acid being constant, the bases to saturate them will be 

 equally constant. 



94. A beautiful chemical law governs this saturation of 

 the vegetable acids. It is the law of substitution, analo- 

 gous in part to the law of isomorphism, or the law of simi- 

 lar forms ; it is perhaps connected with that, so that the 

 elements of isomorphous groups only can be substituted for 

 one another. In minerals which are crystallized it was for- 

 merly thought that similarity of external form indicated 

 identitv of chemical composition. Later observation has 



