164 MANURES. VII. 



The double sodium-calcium phosphate has a constitution 

 similar to tetracalcium phosphate, and like that substance is 

 soluble in ammonium citrate solution (up to 95 % of the total 

 phosphoric acid). The triple silicate of sodium, potassium, and 

 aluminium contains its potash in a more readily available form 

 than the original felspar. The composition of the residue, 

 which is easily powdered, is* 



% 



Phosphorus pentoxide ... 27'01 



Silica ... .. ... 9-99 



Sulphur trioxide ... ... 0;27 



Potash ... ... ... 1-54 



Soda ... ... ... 14-69 



Lime ... ... ... 38-12 



Magnesia... ... ... 2'88 



Iron oxide and alumina ... 4*50 

 Fluorine and loss . 1-00 



100-00 



It is probable that a recently devised, continuous process of 

 dephosphorising pig-iron (Talbot's patent) may lead to an in- 

 creased production of basic slag. 



III. POTASH MANURES. Potash is much more widely 

 distributed and less frequently deficient in soils than are nitro- 

 gen and phosphoric acid. Potash manures, therefore, are less 

 often used than those already described; although certain 

 crops, e.g., potatoes, are greatly helped by their application. 

 Formerly the chief source of potash, both for manurial and 

 other purposes, was the ash left when twigs, leaves, &c., of 

 plants are burnt. Potash occurs in such material as carbonate, 

 and plant ashes are still used to some extent as a manure. 

 They contain the other mineral ingredients, phosphates, lime, 

 &c., but they are especially rich in potash. The chief source 

 of potash is now the immense saline deposits at Stassfurt, and 

 other deposits of a similar kind have been discovered in 

 Thuringia, Brunswick, and Mecklenburg. 



These deposits, in some cases hundreds of feet in thickness, 



* Nilson, Jahresbericht Agric.-Chem. 1899, 127. 



