240 SYSTEMS OF PERMANENT AGRICULTURE 



to render the phosphorus 1 more soluble, by treatment with sulfuric acid. Prac- 

 tically all these attempts have been abandoned, and the only process through 

 which the slag is passed is that of grinding. This must be thoroughly done, 

 for it is found that the availability of the phosphorus depends very largely upon 

 the fineness of subdivision. A sample should contain at least 80 or 90 per cent 

 of powder which passes through a sieve of 100 meshes to the linear inch, i.e. 

 10,000 to the square inch. Thomas phosphate has given excellent results, 

 especially in soil somewhat deficient in lime and rich in organic matter." 



The total quantity of basic slag phosphate now used in Europe 

 as a phosphorus fertilizer amounts to several million tons a year. 



Ground natural rock phosphate has not been put to direct use 

 as a fertilizer to any large extent, but the subject merits and re- 

 ceives a thorough consideration in the following pages. Numerous 

 trials both in Europe and America extending over only one or 

 two years, without addition of organic matter, and in direct com- 

 parison with acid phosphate or bone meal containing an amount 

 of phosphorus equal to the total amount in the raw phosphate 

 used, have not, as a rule, given satisfactory results, and in conse- 

 quence the direct use of this material has been discouraged by 

 some investigators, and the Association of German Agricultural 

 Experiment Stations has even passed formal resolutions discourag- 

 ing the general use of nonacidulated rock phosphate (Landwirt- 

 schaftlichen Versuchs-Stationen, 67, 329). 



The mineral phosphates differ from bone, in that they lack the 

 organic matter in porous structure; and they differ from slag in 

 that they are not mixed or combined with caustic lime capable of 

 slacking and disintegrating into extremely small particles. The 

 fact is, however, that wherever fine-ground natural rock phosphate 

 has been used liberally ; that is, somewhat in proportion to equiva- 

 lent values in comparison with acid phosphate, and in connection 

 with decaying matter, it has given satisfactory results, even during 

 the first rotation, and, with continued use, it proves to be the most 

 economical and profitable form of phosphorus to use in the adop- 

 tion of systems of permanent agriculture. 



In the study of this extremely important question it is well to 

 keep in mind some broad fundamental facts. Thus, the phos- 



1 Substituted for "phosphoric acid," both here and in several other quotations 

 from different writers. C. G. H. 



