SOIL AS THE SOURCE OF OUR MATERIALS 67 



(most of them too small to be seen without a microscope) 

 whose activities have an important bearing upon the life of 

 the green plants that interest us. We have seen that some 

 of these microbes are useful, as in the case of the bacteria 

 living in the tubercles of clover and alfalfa etc. (see p. 62). 

 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. 

 Another effect of fertilizers has been shown in relation to 

 the fact that growing plants, like other living things, throw 

 off waste matters. Some of the waste matters thus thrown 

 into the soil are poisonous. Certain materials added to soil 

 containing these poisons have been found 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 produced by the 

 usual inhabitants of the soil that we <Io not often see. 



97. Intensive cultivation. But even if, by using fertilizers 

 and other substances, we are able to keep the soil under culti- 

 vation indefinitely, the pressure of the population must appear 

 as soon as all the suitable farm land is settled. Modern science 

 has anticipated this emergency by teaching us how to get more 

 food out of every acre of land, through what is called inten- 

 sive farming. This includes a thorough use of the soil through- 

 out the year. By forcing plants to grow more rapidly than 

 they would ordinarily, by selecting rapidly maturing varieties, 

 by covering against cold weather, by artificial watering, by more 

 thorough tilling, and so on, the cultivator is enabled to pro- 

 duce from two to seven crops a year on a given piece of land. 

 This makes possible the support of a larger population on the 

 same territory. 



98. More soil. In addition to making the soil under culti- 

 vation yield more, a civilized people is able to extend its 

 resources in other ways. In this country, for example, nearly 



