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NUTRITION 



are decomposed, and some combine with the oxygen in the 

 air, and pass off in the form of gases (carbonic acid, ammonia, 

 water, etc.). There remains behind only a little mass of solid 

 called the ash. Examining the gases which pass off and the 

 ash which remains, we learn that the following elements are 

 always present in plants: — Carbon (C), Hydrogen (H), Oxygen 

 (O), Nitrogen (N), Sulphur (S), Phosphorus (PV Potassium 

 (K), Magnesium (Mg), Calcium (Ca), Iron (Fe). [Sodium 

 (Na), and Chlorine (CI).] Occasionally other elements also 

 occur. These elements are combined to form the various 

 chemical compounds of which the plant is composed. The 

 grown-up plant contains a larger quantity of each of these 

 elements than did the seed from which it developed. The 

 growing plant, therefore, must have absorbed these elements 

 from the soil or from the atmosphere. 



4. Composition of the Air and of the Soil. — The atmos- 

 pheric air consists mainly of free nitrogen (N)* and free 

 oxygen (O), very little carbonic acid (CO 2), traces of ammonia, 

 (NHg), and water vapour (HgO). 



The soil is mainly made up of small particles or grains, 

 amongst which there are little spaces occupied by air and 

 water. The grains of soil contain salts which are soluble in 

 water : thus the water in the soil is not pure, it is a solution 

 of certain salts. These soluble salts contain all the elements 

 required by the plant. In addition, the soil contains the 

 decaying remains of dead plants and animals, or humus. The 

 accompanying table shows which are the commonest inorganic 

 salts dissolved in the water of the soil. We particularly note 

 the absence of carbon from this list — 



* The element Argon is included under the head of Nitrogen, because 

 we do not know its relation to plant life. 



