226 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 11)02. [bull. 213. 
The possible commercial value for agricultural uses of these high 
phosphorus slags produced by the Thomas- Gilchrist process was early 
recognized. At first, however, it was argued that in an untreated con- 
dition they would be useless as fertilizers; that the phosphoric acid 
they contained was not directly available for plants, and that the fer- 
rous oxide would probably prove positively injurious to vegetable life. 
Attempts were accordingly made to dissolve out the phosphates of the 
slag and reprecipitate them. Fortunately this treatment was soon 
shown to be unnecessary, for field experiments with finely ground but 
otherwise untreated slags proved that they were excellent fertilizers. 
From this date (1882) onward the use of Thomas slag as a fertilizer 
has increased steadily, and it is now an important article of commerce. 
Regarding the chemical composition of these slags, facts of great 
economic importance were brought out by the work of Ililgenstock 
and later investigators, and the efficiency of Thomas slags as fertiliz- 
ing agents is now explained. In rock phosphates the phosphoric acid 
exists combined with lime as the tribasic lime phosphate (3Ca() 3 P 2 5 ). 
In the slags above mentioned, however, the combination existing is 
the tetrabasic lime phosphate (4Ca0 3 P 2 5 ). These two compounds 
differ greatty in the degree of their solubility in saline solutions, the 
tetrabasic phosphate being much more soluble than the tribasic. For 
this reason the phosphate slags are more efficient as fertilizers than 
the mineral phosphate. Jerisch states that about U percent of the 
total phosphorus of slags is present in the form of phosphide of iron, 
which is changed into phosphate in lie soil. 
Many types of crushers and mills have been experimented with in 
the pulverizing of Thomas slag. The ball mill, however, seems to be 
the only one capable of economically crushing this product to the 
fineness required — 75 per cent through a 100-mesh sieve. 
The slight development of the basic Bessemer steel industry in the 
United States necessarily renders the use of these phosphatic slags of 
less commercial importance than in Europe. During the year 11)01 
about 1,000 tons of phosphate slag, produced in the United States, 
were sold as fertilizer. This American material has been tested b} r 
the Maryland Agricultural Experiment Station, the report a of the 
results being that slag phosphate gave a greater total yield than did 
any of the other insoluble phosphates. The yield of corn with slag- 
phosphate was not quite so much as with bone meal, but the yield of 
wheat and of grass was greater. All yields were produced at less cost 
with slag phosphates than with bone meal. The slag used in these 
experiments was a commercial sample and contained 16.32 per cent 
total phosphoric acid. Other commercial analyses of this fertilizer 
show phosphoric acid contents of 21.03 and 22.24 percent. A com- 
plete analysis of the slag from the Pottstown, Pa., converters is given 
in the preceding table of analyses. 
a Bull. Maryland Agric. Exp. Sta. No. 68, p. 28. 
