1^\CAi 



CHAPTER XV 

 THE SOIL AS THE SOURCE OF OUR MATERIALS 



93. The soil. Just as the sun-light and sun-heat are the 

 sources of our energy, so the materials in the soil, water, and air 

 are the sources of our physical constitutions. We have seen 

 that water is necessary for all life processes, and that the 

 carbon dioxid in the air supplies material for the making of 

 carbohydrates. All the other materials found in the bodies 

 of plants and animals are derived from the soil.^ Those who 

 live in the country usually appreciate our dependence upon the 

 soil for our life, but most young people in the cities come to 

 think of the land as merely the place or surface upon which 

 we live. The crowding of a population may mean not simply 

 that people live too close together for comfort or for health ; 

 it may mean also a shortage of food supply due to insufhcient 

 soil for growing crops. 



As the population of a nation grows, the second kind of 

 crowding is likely to become serious. There was a time when 

 thoughtful people looked forward to such overcrowding with 

 a feeling that it must result in great destruction of human 

 life or in great suffering through general poverty. Indeed, in 

 past times much of the poverty and famine was due to the 

 inability to secure from the soil adequate supplies of food. At 

 the present time, however, we are rapidly learning to increase 

 the yield of our cultivated land out of proportion to the increase 

 in population. 



^ This is apparently not true of the plants that float in ponds or rivers or 

 oceans, and of the animals that feed upon such plants. But in reality the 

 salts dissolved in the waters have been washed out of the rocks and soils 

 along which the brooks and rivers flowed on their way to the sea. 



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