MANURES AND MANURING 1 75 1 



addition of salts of acids weaker than citric acid owing to the fact that 

 in such cases {e. g. carbonates) the citric acid displaces any such acid ra- 

 dicle and loses part of its solvent power in the process. 



Assuming that the reactions in the soil are analogous to those described 

 above, the following practical conclusions may be drawn : i) Fertilizers 

 containing calcium salts lower the availability of phosphates ; 2) nitrogen 

 and potash manures in the form of sulphates, nitrates and chlorides increase 

 the availabiHty of phosphates in the order given ; 3) magnesium sulphate, 

 nitrate or chloride acts in a similar fashion. 



Calcined and basic phosphates. — The writer states that the calcining 

 process merely raises the cost of the product without rendering the phos- 

 phoric acid more soluble. 



With regard to the production of basic phosphates, so far the only 

 process working on an industrial scale is in Sweden where 20 000 tons 

 of « Wiborgh « phosphate are turned out per annum. Ground apatite is 

 heated with alkahne sulphate to a red or yellow heat and the product 

 contains phosphoric acid which is soluble in 2 per cent citric acid. It has 

 been thought that the tricalcic phosphate combines with the alkali at 

 high temperatures to form a tetracalcium phosphate (5CaO. 2N02O. 

 4P2O5 or 5CaO. 2K2O. 4P2O5) analogous with the one beheved to exist 

 in basic slag. The writer, however, does not accept the view that this com- 

 pound exists either in Wiborgh phosphate or in basic slag, and believes 

 that the increased quantity of soluble phosphoric acid in the former is due 

 to the presence of alkahne sulphates. 



Basic slag. — A series of experiments was undertaken to throw light 

 on the question as to whether the solubility of the phosphoric acid in basic 

 slag be due to some of the secondary products contained in the slag. The 

 raw phosphates were treated with citric acid in the presence of : iron and 

 aluminium fiUngs, ferrous sulphate, ferric chloride, aluminium sulphate, 

 manganese sulphate and hydrate and magnesium silicate. The following 

 results were obtained : 



i) Silicates exerted a retarding effect on the solubility of the phos- 

 phoric acid, silicic acid being weaker than citric acid. Manganates behaved 

 in a similar manner. 



2) Ferrous sulphate and manganese sulphate behaved exactly hke 

 ammonium sulphate, alkaline sulphates and magnesium sulphate, the 

 solubility of the phosphoric acid increasing with the concentration of these 

 salts. 



3) The same is true in the case of ferric and aluminium salts the pre- 

 sence of a very small quantity of these salts being sufficient to increase 

 markedly the solubility of the phosphoric acid. It is stated that the increased 

 solubility here is due to the combined effect of two distinct causes : a) the 

 retarding of the equilibrium by the slag, and b) the formation of a complex 

 salt of iron or aluminium with citric or phosphoric acid. This complex salt 

 undergoes little dissociation and causes a radical change in the equilibrium 

 of the system : tricalcic phosphate, phosphoric acid, calcium citrate, ci- 

 tric acid and ferric or aluminium salts. The fact that similar results were 



