FERTILITY AND SOIL LIFE 107 



Fertilizers. Formerly it was thought that when fer- 

 tilizers were added to soil* the plants simply used the 

 fertilizers directly as "food." But now we have found 

 that fertilizers may produce their good effects without 

 acting as food at all. They may cause the destruction 

 of poisonous substances which are in the soil, or they may 

 stimulate the growth of micro-organisms which are essen- 

 tial to its fertility, or they may simply change the chem- 

 ical condition of the soil in a way that is favorable to plants. 

 Thus fertilizers benefit the soil in various ways, and prob- 

 ably will be used even more in the future 

 than in the past. In the future, how- 

 ever, we may expect that the exact ef- 

 fects which they produce will be better 

 understood, and this will lead to a more 

 scientific and more economical use of 

 them. 



Soil Fertility and Food-Supply. ^l 



The whole question of soil fertility is of FlG - |~ 

 huge importance to every one of us; the 

 future welfare of the human race appears to depend upon 

 it more than upon any other single thing. As the years 

 have gone by, the fertility of many of our greatest agricul- 

 tural regions, especially in the United States, has been 

 decreasing; the amount of crop per acre has been getting 

 less. Meanwhile each year the population of the world 

 increases. Each year there are more mouths to be fed, 

 more hunger to be satisfied. Are we sure that each year 

 there will be more food? Most of the fertile land in the 

 world is already under cultivation, and if fertility continues 

 to decrease while food-consumption continues to increase, 



