No. 4.] CHEMICAL AND FARM MANURES. 137 



Phosphoric Acid. 



Phosphoric acid for agricultural and other purposes is 

 now derived from various materials, though formerly the 

 supply was practically drawn from bone. 



Bone phosphate is a combination of one equivalent of 

 phosphoric acid with three of lime. A similar combination 

 of phosphoric acid and lime is found in the pebble and nod- 

 ular phosphates of South Carolina and Florida, the phos- 

 phates of Tennessee and Pennsylvania, as well as those of 

 Belgium, Algiers and many of those of other countries. 

 Apatite diflers somewhat from those just mentioned, and is 

 generally considered as less assimilable. 



Until of late bone has been almost universally considered 

 as being infinitely superior as a phosphatic manure to any 

 of the materials just enumerated. Recently however, pot 

 experiments conducted by P. Wagner,* and others by 

 SteiFeck and Maercker,| have given indications that the 

 phosphoric acid of bone meal, either steamed or raw, is 

 practically no more assimilable by plants than that of certain 

 finely ground mineral phosphates ; and they conclude that 

 the high value heretofore usually attributed to bone has 

 been due as much, or more, to the nitrogen which it contains 

 as to the phosphoric acid. In the earlier field experiments 

 by Marek } a striking advantage of bone meal over certain 

 forms of mineral phosphate was but infrequently noticed. 

 In a recent contribution to the subject by the veteran agri- 

 cultural chemist, Julius Kuehn§ of Halle, the ground is 

 taken that upon light, sandy soils bone meal may be used 

 even more efiectively than superphosphates (bone, bone- 

 black or mineral phosphate treated with sulfuric acid) ; 

 while upon heavier, clayey soils, he places but small value 

 upon bone as a source of phosphoric acid, as compared with 

 superphosphate . 



In Kiihn's experiments with the sandy soil he employed 

 potash as kainite and nitrogen in the form of sulfate of 



* Die rationelle Dongung der landw. Kulturpflanzen. Darmstadt, 1891. 

 t TJeber die Phosphorsaurewirlcung der Knochenmehle. Berlin, 1895. 

 X Ueber den relativen Diingerwerth der Phosphate. Dresden, 1889. 

 § Berichtaus dem physiologischen Laboratorium v. d. Versuchsanstalt des Landw. 

 Instituts der Universitat Halle. Dreizehntes Heft. Dresden, 1898. 



