794 EVOLUTION AND CONSHRVATION Part VI 



avoid waste of the product. Coal can be burned once; the products of the fire 

 do not return to coal again. What a diflfcrence in the heating bills if oil or coal 

 could be reburned! What a difference when atomic energy can be turned to 

 peaceful ends! 



Renewable Resources. Soil, water, air, and living organisms of all kinds are 

 renewable resources. Air and water can be used over again; soil and living 

 organisms are in certain ways renewable. In one or another situation, all of 

 these need care in order to preserve their greatest usefulness; air needs the 

 least; soil and living organisms the most. There are excellent books that deal 

 with the earth's natural resources, with definite methods, e.g., of keeping 

 streams clean enough for fishes, and of guarding the trees in house lots as well 

 as forests. There are books that deal with the extraordinary increase in human 

 populations of the earth and its relation to space and other possessions and to 

 war. A few are mentioned in the Reading List for this chapter. 



Only one natural resource, the soil, may be further mentioned here. It is 

 one of the most important and rapidly disappearing resources of them all. 

 Natural soil is made of particles of weathered rocks mixed with organic matter 

 — the scattered substance of dead plants and animals intimately associated 

 with living ones, myriads of bacteria, roots searching for water, and burrowing 

 animals, microscopic and otherwise. Such soil occurs only in the shallow upper 

 layers of the earth's crust. It is the fertile layer that pulsates with daily changes 

 of temperature and activity of life, and the deeper changes of seasonal tempera- 

 ture and moisture, and animal migrations. There are chemical cycles of dearth 

 and abundance of a given substance, e.g., perhaps calcium compounds weath- 

 ered from limestone and transported by water. Calcium may be picked up by 

 roots, locked in the plant for its lifetime, then returned to the soil from the 

 dead and softened tissues. Other substances come and go — carbon, nitrogen, 

 sulfur, and others. Soil formation is carried on by the energy of the sun and 

 secondarily by the energy liberated from weathering rock and broken tissues. 

 This fountain of energy flows upward from the fertile soil through the plants 

 that grow out of it, from the insects that live upon the plants, through the birds 

 and rodents that feed upon the plants and insects, and on into the carnivores — 

 shrews that devour insects, and cats that eat field mice. This upward stream of 

 energy flows through a chain of food. It returns to the soil in the byproducts 

 of living and in the dead bodies of plants and animals. 



By natural methods, it takes hundreds of years to make an inch or two of 

 fertile topsoil. By human means, it takes work and money and years, more in 

 some regions than others. It is estimated that since farming started in the 

 United States one third of the whole area of topsoil has been lost, overworked, 

 carried by wind, washed into the rivers, and taken into the sea. Under the good 

 topsoil, there is another layer of soil, poor but present. Land may be danger- 

 ously hurt; but not finally destroyed. Conservation of soil is an effort to renew 

 its pulsating energy. 



