6o ELEMENTARY BIOLOGY 



water and minerals. The water is replaced by the rains, but 

 the minerals are limited in amount. Although the movements 

 of water in the soil may distribute salts and make new quanti- 

 ties available, the exhaustion of certain minerals from soils 

 under cultivation is an established fact. To take the nitrates 

 •alone, we find that the plants can manufacture proteins only 

 if they have a supply of nitrogen in a combined form ; that is 

 to say, they cannot utilize free nitrogen, such as is found in 

 the atmosphere, but must have nitrogen compounds. 



In the course of the vital processes in the bodies of plants 

 and animals, proteins are broken down into simpler compounds 

 of nitrogen. Some of these can be used again by plants in the 

 making of proteins, but others disappear in the air, and so the 

 nitrogen is lost from the cycle of life. As a matter of national 

 economy, people are finding it worth while to save the manure 

 of the barnyard and even the sewage of cities for the combined 

 nitrogen that these substances contain. But in spite of all our 

 saving, vast quantities of nitrogen are washed out to sea or 

 thrown into the air beyond the reach of our common plants. 



Resort has been made to deposits of nitrates found in the 

 soil, but the quantity of these nitrates is limited and they are 

 relatively expensive. On certain islands off the coast of .South 

 America there are extensive deposits of guano, or bird refuse, 

 left there by countless birds that have built their nests upon 

 these islands for hundreds of years. This guano contains 

 nitrogen and other elements usable for food-making by plants, 

 and has been imported and sold as a fertilizer. But the amount 

 of guano is limited and constantly diminishing. Indeed, of all 

 the elements, nitrogen seems to be the one that does not come 

 back into the life cycle by an automatic process. From the point 

 of view of a nation that can look ahead more years than the 

 length of an individual's life, this is a serious problem. The 

 nitrogen supply will probably last as long as you and I are likely 

 to live. But society expects to outlive us, and it is the business 

 of the statesman to look ahead for those not yet born (Fig. 20). 



