72 FOOD FKOM THE SOIL 



151. THE FOOD MATERIALS. We have seen that all 

 food materials must be in solution in water in order to be 

 taken in by the roots. Different kinds of plants require 

 different kinds and proportions of the food materials, but 

 ordinary green plants are supposed to require at least 

 eleven of the elementary substances in order to live. 

 These are : 



Carbon, C. Potassium, K. 



Oxygen, O. Calcium, Ca. 



Nitrogen, N. Magnesium, Mg. 



Hydrogen, H. Phosphorus, P. 



Sulfur, S. 

 Iron, Fe. 

 Chlorine, Cl. (in some 



plants) . 



All these elements must be in combinations, not in their 

 elemental form, in order to be absorbed by roots. 



152. Usually all of these except carbon and oxygen are 

 taken in only through the roots. Some of the oxygen is 

 taken in by the roots in the form of water (which is EbO), 

 and in other compounds. Some carbon is probably taken 

 in by the roots in the form of carbonates, but it is doubt- 

 ful whether this source of carbon is important to the plant. 

 Water is not only a carrier of plant -food: it is itself a 

 plant-food, for some of it is used in the building up of 

 organic materials. The seven elements in the right-hand 

 column are called the mineral elements: they remain in 

 the ash, when the plant is burned. The mineral elements 

 come from the soil. 



153. The ash is a small part of the total weight of 

 the plant. In a corn plant of the roasting -ear stage, the 

 ash (what remains after ordinary burning) is about 1 per 

 cent of the total substance. 



J54. Water is the most abundant single constituent 

 or substance of plants. In the corn plant of the roasting- 



