82 



SOIL AND ITS RELATION TO ROOTS 



posed to plant corn, the nitrogen left in the soil thus giving nourish- 

 ment to the young corn plants. In scientifically managed farms, 

 different crops are planted in a given field on different years so that 

 one crop may replace some of the elements taken from the soil by 

 the previous crop. This is known as rotation of crops. 1 The 

 annual yield of the average farm may thus be greatly increased. 



Five of the elements necessary to the life of the plant which 

 may be taken out of the soil by constant use are calcium, nitrogen, 

 phosphorus, potassium, and sulphur. Several methods are used 



by the farmer to prevent the 

 exhaustion of these and other 

 raw food materials from the soil. 

 One method known as fallowing 

 is to allow the soil to remain 

 idle until bacteria and oxidation 

 have renewed the chemical ma- 

 terials used by the plants. This 

 is an expensive method, if land 

 is dear. The most common 

 method of enriching soil is by 

 means of fertilizing material 

 rich in plant food. Manure is 

 most frequently used, but many 

 artificial fertilizers, most of 

 which contain nitrogen in the 

 form of some nitrate, are used, 

 because they can be more easily transported and sold. Such are 

 ground bone, guano (bird manure), nitrate of soda, and many 

 others. These also contain other important raw food materials 

 for plants, especially potash and phosphoric acid. Both of these 

 substances are made soluble so as to be taken into the roots by 

 the action of the carbon dioxide in the soil. 



The Indirect Relation of this to the City Dweller. All of us 

 living in the city are aware of the importance of fresh vegetables, 



1 That crop rotation is not primarily a process to conserve the fertility of the 

 soil, but is a sanitary measure to prevent infection of the soil, is the latest belief 

 tf the scientist. 



Nitrogen in the soil is necessary for plants. 

 Explain from this diagram how nitro- 

 gen is put into the soil by some plants 

 and taken out by others. 



