The Composition and Use of Fertilizers. 195 



very largo, as can be inferred from the character of the few re- 

 ferred to. 



Classification into Air-Derived and Soil-Derived Elements. — 

 The elements necessary to the growth of plants may be divided 

 into two quite distinct classes, which have important and marked 

 differences. These two classes are: (a) Air-derived elements. (&) 

 Soil-derived or mineral elements. 



(a) Air-Derived Elements. (&) Soil-Derived Elements. 



Carbon, Potassium, 



Oxygen, Phosphorus, 



Hydrogen, Calcium, 



Nitrogen. Magnesium, 



Sulphur, 



Iron, 



Chlorine, 



Sodium, 



Silicon, 



Manganese. 



It is usual among writers on agricultural chemistry to call these 

 classes organic and inorganic elements, but this use of these words 

 is extremely inaccurate; for any element may be either organic or 

 inorganic, according as it is or is not a part or product of an or- 

 ganized body. Oxygen, as it exists in the air, is inorganic matter; 

 but when, through vital processes, it becomes part of an animal 

 or plant, it is organic. 



These two classes of elements differ in three important par- 

 ticulars, as follows: 



1. The elements of the first class are derived exclusively from 

 the air, either directly or indirectly; while those of the second 

 class come exclusively from the soil. 



2. Air-derived elements disappear, for the most part, in the 

 form of gases, when a plant is burned; while the soil-derived ele- 

 ments, usually the smaller part, are left in the form of a residue 

 or ash, upon which further heating will not have any effect. 

 Some carbon and oxygen and nitrogen are always found in the 

 ash, while slight quantities of chlorine, sulphur and phosphorus 

 are apt to be driven off by heating. The two classes of elements 

 are, therefore, not so sharply defined in this regard as they are 

 in respect to the sources from which they come. 



3. These two classes differ very noticeably in regard to the 



