CHAPTER III 



THE ABSORPTION OF MINERAL ELEMENTS 



22. The Ash Constituents of the Plant. Their Composition 

 and Origin. The Indispensable and Minor Mineral Elements. — 



One of the principal methods of analysis of the plant consists in 

 burning it. During this process the carbon is determined as carbon 

 dioxide, hydrogen and oxygen in the form of water, and nitrogen 

 in the form of molecular N. After the burning of the dry sub- 

 stance of a plant, there always remains a non-volatile residue, the 

 ash. This, too, represents an indispensable part of the living sub- 

 stance. 



The quantity of ash in different parts of a plant is variable. 

 The least amount, about 1 per cent, is found in the wood of trees, 

 which consists primarily of the walls of dead cells. Seeds contain 

 about 3 per cent of ash, the main vegetative organs, stems and 

 roots of grassy plants, 4 to 5 per cent, while the leaves may have 

 as much as 10 to 15 per cent. It is of interest to note that a rela- 

 tively high concentration, approximately 7 per cent, of ash is 

 found in the bark of woody plants. Tissues consisting mainly of 

 living cells are usually richest in ash. It must be understood that 

 these are approximate figures. They will vary not only with dif- 

 ferent plants, but even within the same organs of a species the 

 quantity of ash can fluctuate considerably. Leaves of potatoes, 

 for instance, may contain from 5 to 13 per cent of ash; those of 

 beets, 11 to 21 per cent; and of rape, 8 to 15 per cent. The ash 

 content depends largely on the composition and moisture content 

 of the soil. In general, one may say that the richer in salts the 

 soil is and the dryer the climate, the more ash accumulates in the 

 plant, although a direct proportionality may not always be 

 observed. 



The composition of the ash is very complex and variable. 

 Almost all of the elements, including the rarest ones, have been 

 found in the ash of some plant. Many of the rare elements are 



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