THE WORLD'S FOOD SUPPLY 245 



263. The supply of nitrogen. It is a common practice to 

 return to the soil in the form of manures as much as pos- 

 sible of the straw and other materials which have come from 

 the fields, and in this way much may be done to maintain the 

 fertility of the soil. There is always a loss, however, in the 

 material that is sold and taken off the farm, and there are 

 certain other unavoidable losses of nitrogen. A very impor- 

 tant source of nitrogen is found in the action of certain kinds 

 of bacteria which live in the soil or in the roots of the clover 

 and other plants of the pea family. As stated in section 243, 

 their value lies in the fact that they cause the nitrogen of the 

 air to combine with other elements in such a way as to form 

 the soluble nitrogen compounds which other plants can absorb 

 and use. It is common to restore the nitrogen supply of land 

 by growing clover on- it. 



If the use of clover and its bacteria does not suffice to re- 

 store the land, it is possible to scatter over it certain nitrogen- 

 containing chemicals which may be purchased for this use. 

 The substance used most commonly is Chile saltpeter, which is 

 mined in great quantities in Chile and exported by shiploads. 



264. The value of the soil. It is not alone the farmer who 

 is interested in the soil, for the production of foodstuffs for 

 all of us depends upon the productivity of the soil. If it is 

 generally lacking in the substances and conditions necessary 

 for plant growth, there will not be enough food produced to 

 supply the needs of the country, and all will suffer either in 

 the prices paid or because of lack of food. 



265. Summary. The food of the world is produced by 

 plants. They make carbohydrates from carbon dioxide and 

 water, and fats are made from carbohydrates. Proteins are 

 made by plants and animals by the addition of nitrogen, 

 sulphur, etc. to the elements of the carbohydrates. The 

 former are secured from compounds which are absorbed from 

 the soil. The problem of keeping up a proper supply of these 

 compounds in the soil is a very important one. 



