Commercial Fertilizers. 



155 



to be more available to the plant than the insoluble or natural 

 phosphate ; hence, the soluble and reverted phosphoric acids taken 

 together are known as the available phosphoric acid. -7 



Phosphate rock. This substance has already been mentioned 

 under insoluble phosphate of lime. Rock phosphate is designated 

 usually by the locality from which it is obtained, as: South 

 Carolina rock, Florida rock, Tennessee rock, etc. It contains 2f> 

 to 30 per cent of phosphoric acid and furnishes the chief source 

 of the supply found on our markets. Apatite is a purer mineral 



Florida rock-phosphate mining. 



phosphate and it found in considerable quantities in Canada, 

 Norway, Sweden and Spain. Mention has already been made of 

 the finely ground rock phosphate known as "floats." Recent 

 investigations indicate that when this material is added to farm 

 manure it has a high fertilizing value ; in fact the increased crop 

 production at the Ohio Experiment Station, due to adding ground 

 rock phosphate to stall manure was nearly as large as that ob- 

 tained from the addition of super-phosphate. It would seem 

 from these experiments that the comparatively inexpensive floats 

 might, partially at least, replace super-phosphate, if used in con- 



