46 AGRICULTURAL ANALYSIS 



States in 1906 was 2,080,957 long tons, valued at $8,579,437. 

 A comparison of the figures of late years indicates that, although 

 the output has usually increased each year, the demand has 

 made even more rapid strides, and that the tendency of the 

 market price is upwards. The new Western fields will probably 

 help to supply the increasing demand, but as their market is 

 somewhat local they will not materially affect the general con- 

 ditions throughout the country. The demand will be likely to 

 continue in excess of the supply unless the new Tennessee field 

 proves to be more extensive than is anticipated. The outlook 

 for the newer fields is therefore bright and will soon become 

 even more promising as the older fields become exhausted. It 

 is not impossible that the increasing demand and higher prices 

 will make it possible to operate many low grade deposits which 

 it has hitherto been impracticable to utilize. 



52. Quantity of Phosphoric Acid Removed by Crops. It is esti- 

 mated that the quantity of phosphoric acid removed from the soil 

 annually in the United States is equivalent to that contained in 

 7,000,000 tons of 14 per cent, superphosphate. 24 This estimate 

 does not include the quantity removed by erosion and leaching. It 

 is evident that in order to maintain the present fertility of our 

 arable soil in respect of phosphoric acid about one million tons of 

 this substance, calculated as P 2 O 5 , must be added to the soil each 

 year. 



53. General Conclusions. From the study of the origin of the 

 deposits of mineral phosphates it appears that those which are 

 suitable for economic uses have been derived chiefly from the 

 decay of organic matter mostly of animal origin. The phos- 

 phoric acid was evidently very generally diffused in the mineral 

 matter which first formed the crust of the earth. It began to be 

 utilized by the simplest forms of vegetable growth which first 

 appeared on the earth's surface, and through this intermediary 

 passed into animal organisms. In these it was finally segregated 

 in the bones in large quantities. In the decay of animal bodies 

 the bony structure is attacked by solvents ; for instance, the nitric 

 acid produced by the oxidation of the protein of vegetable and 



24 Voorhees, Journal of the Franklin Institute, 1905, 160 : 211. 



