THEORY OF SOIL FERTILITY 183 



land, the resources of the soil for food produc- 

 tion. The Geological Survey was able to fur- 

 nish in definite form the extent of our coal 

 supply in tons, and from these figures it was 

 simple enough to ascertain the remoteness of 

 the day of doom in coal, by dividing the total 

 supply by the estimated annual consump- 

 tion. They were able to calculate the extent 

 of the national resources in waterpower, 

 measured in feet-per-second of available 

 streams. The Department of Agriculture 

 produced statistics showing that to-day our 

 annual consumption of lumber is forty billion 

 feet and compared this with the annual growth 

 of our forests, showing that if eventually we 

 did not resort to reforestation on a grand scale, 

 we must soon arrive at the end of our supply 

 of timber. They did not concern themselves 

 in this connection with the resources of "plant- 

 food" necessary to grow a definite amount of 

 timber annually. That seemed beside the 

 question. Still, it is readily computed, for we 

 know how much "plant-food" every board-foot 

 of lumber takes from the soil. The mere fac- 

 tor of rain necessary to supply our annual de- 

 mand for lumber, computed on the equation 

 of 1,000 parts of water for one part of dry 



