10 BULLETIN- 69d, TT. S. I>EPAI{T]VtElTT OP AGEICtTLTtrilE. 



mining- the phosphoric acid present in the roots. Analyses of tur- 

 nips grown on soils treated with various forms of phosphoric acid 

 showed that the percentage of P2O5 in the roots was fairly consistent 

 with the availability of the phosphates as determined by crop yields. 



In determining the trade value of fertilizer materials the Associa- 

 tion of Official Agricultural Chemists ^ classes as available all phos- 

 phoric acid soluble in a neutral solution of ammonium citrate, and in 

 the analyses of basic slag the association- has tentatively adopted 

 Wagner's method, in which all phosphoric acid dissolved from a 

 definite weight of the material by a certain volume of 2 per cent citric 

 acid is considered available to crops. 



Although several of these methods for determining the availability 

 of phosphoric acid both in soils and fertilizers are useful in showing 

 the relative solubility of various phosphates, nearly all of the proc- 

 esses so far suggested are empirical, and none of them are founded 

 on a strictly scientific basis. While it has been demonstrated by 

 actual field experiments that certain phosphates soluble in weak 

 acids and dilute organic solutions are also active under soil condi- 

 tions, in the light of both field and laboratory investigations all phos- 

 phatic materials which do not conform to these tests can not be 

 classed as unavailable to crops. It must be remembered that the term 

 " available " is a relative one. Practically every phosphate known is 

 soluble to a certain extent even in pure water. The amount of phos- 

 phoric acid dissolved from the less soluble phosphates depends on the 

 surface exposed, the quantity of solvent used, and the time of contact. 



Raw rock phosphate conforms in part at least to practically all of 

 the chemical availability tests, but its solubility in the various con- 

 ventional media is materially affected by the three factors mentioned 

 above. No method, therefore, which calls for contact for a limited 

 time and the use of a definite quantity of relatively weak solvent, 

 and does not state the degree of fineness to which the material should 

 be ground, can show any sharp distinction between the amounts of 

 phosphate available and unavailable under the conditions obtained 

 in the soil. 



EFFECT OF FINENESS OF GRINDING. 



The fineness to which phosphate rock is ground undoubtedly has 

 a very important influence on its availability to crops. Not only is 

 it easier to distribute more uniformly the finely ground rock through 

 the soil, but the immense amount of surface exposed by such material 

 enables the soil water to dissolve a much greater proportion in a 

 given time than where the rock is in relatively coarse particles. 



1 Jour. Assoc. Official Agr. Chemists, 1» No. 4, 4-5 (1916). 



2 Idem., pp. 14-15. 



