994 The American Naturalist. [December, 
the life of vegetation the life of animals must cease. The 
whole living world must end. 
When the scientist observed this fact he immediately looked 
around to see if there was not a remedy for it. Now, as far as 
some of the plant foods are concerned, there does not seem to 
be any occasion for fear. The phosphates, the sulphates and 
the potassium salts, which are plant foods, seem to exist on 
the surface of the earth in almost unlimited quantities. There 
have been immense amounts of these salts found in certain 
parts of the world, and they can be mined at very small 
expense; they can be taken all over the world and put directly 
upon the soil, so that the sulphates, phosphates and potassium 
salts are in practically unlimited quantities. We have nofear 
so far as they are concerned. For an indefinite number of 
ages to come there is plenty of this sort of food on the surface 
of the earth for us to supply to the soil. But that is not true 
of the nitrogenous foods. Of course every farmer knows 
to-day that nitrogenous food is one of the very essential foods . 
of plants, and it is not true that there is an unlimited quan- 
tity of nitrogenous salts anywhere in the world. There are@ 
few sources of nitrogen other than the soil. The chief one 18 — 
the guano beds in the South Pacific. These are sources of 
nitrogenous compounds, and upon these sources the agricuk 
Aas 
Hi 
5 
A 
oth Fae ee 
tural industry of the world has been drawing for years, an L 
will continue to draw until they are exhausted. But these 
sources are far away. The nitrogen that we get from them 18 7 : 
very expensive, and the store is very limited in quantity. — 
We can see in the not very distant future the complete exhaus 
tion of all these nitrogen beds. This has led scientists to look 2 
with a considerable degree of dismay upon the future of a 
vegetable world. What is going to happen when all a 
available nitrogen is used up? If we are going to continue © 
take the nitrogen from the soil and throw it into the ocean "Y 
will soon exhaust the soil, and if there is no store of nitrogen 
anywhere for our plants to draw upon what are our plan 
going to do in the future ? " ne: 
Now there is a store of nitrogen in the world which 18 abso- 
lutely unlimited, and thatis in the air that surrounds us. ~ 
