94 BIOLOGY AND HUMAN LIFE 



rock, and many others. In this country the farmers spend over 

 $300,000,000 annually for commercial fertilizers, besides what 

 they use from their own dungheaps. 



The first use of fertilizers is to place in the soil the materials needed 

 for plant growth. Certain fertilizers, however, are sometimes added not 

 to supply material but to produce chemical changes in the soil, to make 

 the latter more suitable for the growth of plants. For example, gypsum 

 is commonly used to supply calcium ; but in some cases it is used to 

 make it easier for the plant to get the phosphorus in the soil. 



71. Biology of the soil. The soil contains many different 

 kinds of very small plants and animals, most of which can be 

 seen only with a microscope. Some of these microbes are useful, 

 as in the case of the bacteria living in the tubercles, or little 

 swellings on the roots, of clover and alfalfa etc. (see page 301). 

 Others, however, are injurious. Some of the latter may be 

 destroyed by the addition of sulfur to the soil, with the result 

 that the size of the crop is increased. Strictly speaking, the 

 sulfur is not a fertilizer, although it helps to increase the yield. 



Growing plants, like other living things, throw off waste mat- 

 ters. Some of these wastes thrown into the soil are poisonous. 

 Certain materials added to soil containing such poisons are 

 helpful, not because they add anything usable but because they 

 counteract the poisonous substances. In a similar way certain 

 materials may help by counteracting the poisons or acids pro- 

 duced by the usual inhabitants of the soil that we do not 

 often see. 



72. Intensive cultivation. By using fertilizers and other sub- 

 stances we may be able to keep the soil under cultivation 

 indefinitely ; yet as soon as all the suitable farm land is settled 

 and cultivated there must be a crowding, or pressure, of popula- 

 tion. Modern science teaches us how to get more food out of 

 every acre of land through intensive farming. By forcing plants 

 to grow more rapidly than they would ordinarily (by selecting 

 early-maturing varieties, by covering against cold weather, by 

 artificial watering, by more thorough tilling, and so on) the 



