4 SOILS AND MANURES 



THE PLANT. 



Composition of Plants. Plants are composed of certain 

 chemical elements. These must be supplied from some- 

 where or the plants cannot be formed. They can be 

 determined by chemical analysis, and the sources from 

 which they are derived can be traced by further experi- 

 ment. The methods of analysis of plants involve numerous 

 complex processes, a detailed description of which lies 

 outside the scope of this work. A brief outline, however, 

 is easily followed, and may help to convey some idea of 

 the properties and relations of the substances referred to. 



If a plant, or quantity of any vegetable matter, be dried 

 as, for example, when grass is made into hay put into 

 a crucible and heated to redness, it takes fire and burns. 

 The great bulk of it disappears and only a small quantity 

 of ash remains behind in the crucible. The part which 

 disappears on incineration is commonly spoken of as the 

 organic matter or volatile portion, and the ash is, by con- 

 trast, known as the mineral or non-volatile matter. These 

 names are apt to be misleading ; the difference between 

 the two kinds of matter implied by them is not real and 

 true. The distinction, however, is a common and con- 

 venient one, as the two portions can be examined and con- 

 sidered separately. 



The relative proportions of ash and organic matter vary 

 in different kinds of plants, in different parts of plants, 

 and even in individual specimens. The table on page 5 shows 

 the proportions in which they are commonly present. 



The part which is volatilised x when the plant is burned 



1 The organic matter is not volatilised as such. The elements of 

 which it is composed except nitrogen, which is liberated in the free 

 state unite with the oxygen of the air, forming volatile oxides. The 

 oxygen found in the products of combustion is therefore mainly 

 derived from the air, but it can be shown that a part of it pre-existed 

 in the plant. 



