SOURCES AND COMPOSITION OF 



305 



Rock phosphate is used to a certain extent as a fertilizer. 

 When ground very fine, so the particles may float in the air, it is 

 known as floats. It does not act as quickly as acid phosphate, 

 and may give little results the first year. Some soil chemists 1 

 advocate the use of heavy applications of ground rock phosphate, 

 together with liberal applications of manure or green crops plow- 



Fig. 72. Mining pebble phosphate. 



ed under, for staple crops like corn. According to the Ohio field 

 experiments, acid phosphate used with manure gives larger net 

 returns than rock phosphate. 2 



Apatite. This is a crystallized calcium phosphate which occurs 

 in quantity in Canada. The highest grade contains 40 per cent. 

 phosphoric acid. 



1 Hopkins, Soil Fertility and Permanent Agriculture, p. 226. 



2 Circular 120, Ohio Station. 



