CHAPTER VII 



ABSORPTION OF MINERAL ELEMENTS BY PLANTS 



55. Mineral or Ash Constituents of the Plant. Their Com- 

 position and Origin. Indispensable and Supplementary Mineral 

 Elements. The Significance of 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 N2. After the burning of the dry substance 

 of a plant, there always remains a nonvolatile residue, the ash. 

 This, too, represents an indispensable part of the living substance. 



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 

 relatively high concentration of ash, approximately 7 per cent, 

 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 

 different plants; 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 J3 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 does not always exist. 



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 

 dispersed in the crust of the earth in such small amounts that they 

 may not be discovered by ordinary analysis, but they may be 



239 



