PHYSICAL STRUCTURE OF THE SOIL 293 



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 316 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 planting it to crops of 

 clover, alfalfa, cowpeas, or other related plants. 



If the use of these plants and their bacteria does not re- 

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

 containing chemicals. The substance which was used most 

 commonly before the world war, and which is still somewhat 

 used, is a nitrate called Chile saltpeter, which is mined in 

 great quantities in Chile and exported by shiploads. Nitrogen 

 and other important plant foods are now secured in large 

 quantities from tankage, dried blood, etc., produced by the 

 meat-packing houses, and from ammonium sulphate, which 

 is produced in the coke ovens. Important processes have 

 been discovered by means of which the nitrogen of the air 

 can be secured and used for agricultural and other uses. 

 Large factories are being constructed for the purpose of 

 preparing commercial supplies of nitrogen compounds by use 

 of nitrogen from the air. 



295. Value of the soil. It is not alone the farmer who is 

 interested in the soil, for upon its fertility depends the pro- 

 duction of foodstuffs for all of us. 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. 



