204 WALL'S MANUAL 



these base3 may be taken up by the plant to the 

 exclusion of the usual proportion of the other ; 

 hence, one base may be substituted for another in 

 equivalent proportion, but through certain elements, 

 may supply the place of others, yet no one element 

 can supply the place of all the others.- Nor can the 

 possible mixture of mere silicates and salts alone give 

 fertility to a barren soil. Fertility depends, in a 

 great measure, upon the presence of vegetable 

 matter in the soil. This vegetable, or organic 

 matter, is constantly undergoing changes, and this 

 chemically induced change exerts a powerful influence 

 upon fertility. 



In the products of the decomposition of organic, 

 or vegetable matter, a variety of substances is 

 formed, differing according to circumstances, the 

 time and progress of decay. In this decomposition 

 of vegetable matter in the soil, the nitrogen it may 

 have contained combines with hydrogen, a highly 

 important fact to be remembered. This union of nitrogen 

 and hydrogen produces ammonia. The nitrogen of 

 the putrifying body is thus converted into ammonia, 

 and there remains the several forms of " humus," or 

 vegetable mold. 



The elements which do not usually enter into 

 the constitution of animal or vegetable matter, 

 mostly from comparatively simple compounds; 

 while the four "organic" elements ^carbon, oxygen, 

 hydrogen, nitrogen though few in number, form 

 an immense number of compounds of great com- 

 plexity, Hence, the moment life departs, the 

 animal or plant speedily undergo new changes ; its 

 elements which life had organised, obey now not the 



