486 Journal of Agricultural Research voi. vi, no. 13 



III. A comparative study of the productive powers of six mineral 

 phosphates for farm crops. 



IV. The influence of fermenting dextrose and crop residues on the 

 availability of phosphorus in finely ground rock phosphate. 



V. The influence of the size of particles on the availabiHty of phos- 

 phorus in mineral phosphates. 



REVIEW OF LITERATURE 



The availability of mineral phosphates for plant nutrition has been 

 under investigation at various institutions for more than half a century. 

 Among the earlier scientists who attempted to determine the availability 

 of the phosphorus in mineral phosphates was Dyer (4), who found that 

 undissolved phosphate produced better returns than dissolved phosphate 

 for swedes and oats. Frear (5) studied the comparative value of various 

 phosphorus carriers for farm crops. Finely ground bone and reverted 

 phosphate produced the largest number of mature stalks of corn, 

 and finely ground bone, the highest yield of ears. Superphosphate 

 and certain mineral or raw phosphates were put in field trials by 

 Johnson (9), and for corn, dissolved bone black was superior to others 

 tested. Bishop (i) grew soybeans in pot cultures and concluded that 

 concentrated phosphate and acid phosphate were more desirable than 

 Florida soft rock and iron and aluminum phosphate. Equivalent 

 amounts of different carriers of phosphate were employed by Hess (7) 

 in a 4-year rotation of corn, oats, wheat, and grass. Finely ground bone 

 gave the highest yields of wheat, with raw rock second. Ground bone 

 was most effective for corn, while for oats insoluble ground bone seemed 

 to be satisfactory. South Carolina rock was very useful for clover. 

 Jordan (10) conducted two experiments at the Maine Station with 

 different forms of phosphate. In the first experiment the minerals were 

 applied in equal quantities. For the first two years the acid phosphate 

 gave the highest returns, but later bone meal took the lead. Raw rock 

 was only about half as productive as the other two. In the second trial 

 equal money values of phosphates were applied; and the author points 

 out that, with but one exception, the raw rock gave larger returns than 

 acid phosphate. The work of Jordan, previously mentioned, was con- 

 tinued by Merrill (15), who used pure sand cultures in the greenhouse. 

 Two facts are clear from Merrill's work. First, plants differ widely in 

 their power to assimilate phosphorus from different phosphates. Second, 

 turnips and rutabagas gave almost as good results with raw rock phos- 

 phate as with acid phosphate. Later, at the New York Station, Jordan 

 (11) continued the work which he had begun at the Maine Station. His 

 results are in accord with the work previously reported by himself and 

 Merrill. 



