.1. F. TociiER 133 



degree of fineness, and containing the same proportions of liydroxy- 

 apatite but quite different proportions of calcium carbonate. The sample 

 wliich contained the smaller (|uantity of calcium carbonate would show 

 a higher citric solubility than the second sample which contained a 

 higher proportion of calcium carbonate, due to the presence of a relatively 

 large proportion of free citric acid in the former. 



Table V. Composition of Mineral Phosphates. 



Cak-iura o.xide 

 Phosphoric acid ... 

 Carbon dio.\ide 

 Moisture ... 

 Combined moisture and) 



organic matter ... \ 

 Ferric and ahiminium j 



o.xides ... ... ( 



Magnesium oxide ... 



Sand 



Undetermined 



Citric solubility is not necessarily a te.st of the availability of the 

 phosphate to the plant in the soil. If the sample is finely ground and 

 has a low citric solubihty, the lowness of the citric solubihty in the case 

 of mineral phosphate would mainly be due ( 1 ) to the presence of alkahne 

 material which would neutralise a large proportion of the citric acid, 

 leaving the residue to act on the phosphate and (2) to the chemical 

 constitution of the phosphatic mineral. In the case of slags the citric 

 solubility would be mainly dependent on ( 1 ) the compounds of fluorine 

 as shown by Robertson, (2) the presence of alkaline lime as shown by 

 Ramsay, and (3) the chemical constitution of the phosphatic compound 

 in the slag. It has yet to be shown that the phosphate in mineral phos- 

 phate is not utiUsed by the plant as readily and as efiiciently as the 

 phosphate from slags. In other words the exact chemical composition of 

 the phosphatic compounds in the various mineral phosphates and slags 

 has, in each case, to be demonstrated. It appears to be necessary to 

 test the citric solubihty of phosphates of knoivn composition against their 

 availability in the soil as shown by yield of crop. The results would then 

 show how far, if at all, citric solubihty is a measure of availability in 

 the soil. Since the commercial fertilisers tested contain varying quantities 

 of alkaline Hme, fluorides and other interfering substances, and since 

 the chemical constitution of the fertihsers is incompletely known, the 

 writer can see no scientific validity in the use of citric solubihty as a 

 measure of availability. The three practical tests appear to be : 



