PLANT FOOD 39 



that light is necessary in order that decomposition may 

 be effected ; further, it has been proved that the process 

 is in some way intimately connected with the green 

 chlorophyl grains which give the characteristic color 

 to leaves. However, the important facts are that the 

 chlorophyl grains assimilate a part of the carbonic acid 

 of the air and that the assimilated product is converted 

 into the various components of the plant. 



Oxygen — This element is a gas and comprises 

 about one-fifth of the air. Oxygen forms a part of 

 almost every compound that plants use as food, e g* 

 water and carbonic acid gas. 



Respiration in plants as well as in animals is sup- 

 ported by the oxygen of the air. The supply of this 

 element, whether in free or combined form, is unlimited 

 and adequate for the needs of plants. 



Nitrogen — Nitrogen is a gas. It comprises about 

 four-fifths of the atmosphere. Grain crops such as 

 corn and oats can flourish only when their roots have 

 access to certain compounds of nitrogen, namely, 

 nitrates and ammonium salts. On the other hand, for 

 leguminous plants it is essential either that nitrates 

 shall be present in the soil, or that certain microscopic 

 organisms having power to fix nitrogen from the air, 

 live upon the roots and accumulate nitrogen upon 

 which the plant can feed. The compounds of nitrogen 

 which the ordinary farm crops use are taken into the 

 plant, after being dissolved, through the roots. A 

 limited amount of nitrogen is found in plants, but it 

 is an absolutely essential element and the supply of it 

 in many soils is quite limited. Moreover, nitrogen in 

 a form available for plants is very liable to waste, 

 being readily carried from the soil with the water of 

 percolation. Therefore, nitrogen is one of the most 



