J. F. Tocher 143 



be a measure (1) of fineness of grinding as already pointed out by other 

 workers, but it seems necessary also to postulate similarity of composition 

 in comparing degrees of fineness in practice, (2) of tlie presence or 

 absence of alkaline substances in fertilisers approximately of the same 

 composition and ground to the same degree of fineness, (3) of the presence 

 or absence of fluorides as well as alkaline substances in slags and (4) of 

 the differences in the constitution of the phosphatic compounds in finely 

 ground fertilisers containing approximately the same proportions of ex- 

 traneous substances. 



None of these conditions are realisable in practice because the propor- 

 tions and the actions of extraneous substances are generally undeter- 

 mined and because the nature of the phosphatic compound present is 

 either unknown or is not given. Further we can with any phosphatic 

 fertiliser irrespective of its composition and constitution, use concentra- 

 tions or dilutions to get any value of citric solubihty we please. Citric 

 solubihty is therefore an unreliable empirical test of the agricultural 

 value of mineral phosphates and slags. It has yet to be .shown that high 

 citric solubihty is a measure of the presence of phosphate in a readily 

 available condition for plant growth, or is an indication of the presence 

 of a highly citric soluble phosphatic compound of a known chemical 

 constitution. We are driven, therefore, to the conclusion that the only 

 practical tests of value of phosphatic fertilisers from the agricultural 

 standpoint are : 



(1) Total phosphatic content. 



(2) Degree of fineness of grinding. 



(3) Freedom from injurious substances and of substances inhibiting 

 plant growth. 



The eft'ect of using dift'erent kinds of pho.sphatic fertilisers on yield 

 of turnip crop, where the same amount of fertihsers, expressed as tri- 

 calcium phosphate, is apphed in each case forms the subject of a separate 

 communication. 



{Received December 23rrf, 1921.) 



