22 BULLETIN 102, PAET 2, UNITED STATES NATIONAL MUSEUM. 
situation so that its significant features will stand forth clearly. 
It is the purpose, in short, of this series of papers to present the 
salient facts of the American mineral industries, without a popular 
understanding of which, it is believed, the greatest progress can not 
be made. 
SUMMARY. 
The theories of soil formation and of plant growth are explained in 
a general way, and it is shown that under prolonged cultivation the 
soil is exhausted in respect of three of its most important plant 
foods — phosphorus, nitrogen, and potassium. 
The chief source of phosphorus supply is phosphate rock, occurring 
in present abundance in the United States. As this material must 
be treated with sulphuric acid to produce fertilizer, its manufacture 
is closely allied to the sulphuric- acid industry. A critical situation 
has arisen in this regard, because Spain is the principal source of 
pryite, from which most of our sulphuric acid is made, and Spanish 
ore shipments have been endangered and largely cut off as an indirect 
result of the submarine campaign. 
The sources of nitrogen compounds are considered and the strong 
diversion of Chilean nitrate to explosive manufacture pointed out. 
The dependence of the United States upon Chile for sodium nitrate 
and the rising prices of organic nitrogen urge the desirability of an 
enlarged by-product coal industry and suitable building up of an 
atmospheric nitrogen industry in this country. 
The dependence of this country upon Germany in respect to potash 
is emphasized and the various war-developed domestic sources of 
potash described. The assistance that the domestic potash industry 
merits at the close of the war is discussed. 
In conclusion, the point is stressed that the best progress in the field 
of fertilizer will come through a true coordination of its various 
parts, both among themselves and together in respect to other indus- 
tries, as the result of enlightened cooperation between the fertilizer 
industry, the Government and the people. 
