ABSORPTION OF MINERAL ELEMENTS BY PLANTS 265 



the ash content of a plant. One cannot conclude from this, 

 however, that accumulation of ash in the plant is merely a 

 mechanical process and simply the result of evaporation of water 

 from the leaves, the salts dissolved in it being left in the plant. 

 Plants growing even close together on the same soil always show 

 differences in ash composition, thus indicating clearly the 

 selective accumulation of some elements in preference to others. 

 Likewise, in different parts of the same plant, the composition of 

 the ash varies considerably. 



The table on p. 264 contains data on the composition of ash 

 of the vegetative organs and seeds of some important agricultural 

 plants. As is customary, the percentages are calculated not on 

 the basis of pure elements, but as their oxides; and the amount 

 of each oxide is given as the percentage of the total weight of 

 pure, carbon dioxide-free ash. 



The table shows that seeds are rich in phosphorus and potas- 

 sium, elements very necessary for the building of new organs and 

 very often lacking in the soil. The stems and leaves of cereals 

 often show a high silicon content, and those of flax and clover, a 

 high calcium content. In the subterranean storage organs, large 

 amounts of potassium are present. This seems to suggest an 

 important role of this element in the storage of carbohydrates. 



Because of the varying ash content and composition and the 

 different capacity of plants to feed upon slightly soluble soil 

 compounds, they naturally possess different requirements for 

 soil nutrients and need, therefore, different fertilizers. Potatoes, 

 for instance, often respond to potash, and cereals to phosphorus. 

 Moreover^ in agricultural practice, various amounts of ash 

 elements are removed with the harvest, and therefore various 

 crops exhaust the soil to a different degree. The average quan- 

 tities of ash elements extracted from the soil by certain agri- 

 cultural plants are given in the following table, the figures 

 indicating the number of kilograms per hectare. 



