184 CHEMICAL FERTILIZERS 



very seriously, but for the purpose of manufacturing ammoni- 

 ated superphosphate that amount of rock phosphate need 

 not be used. It would not be desirable to have more than 

 two-thirds of the total amount of phosphate in an insoluble 

 form. 12 cwt. of superphosphate, 5 cwt. of Gafsa rock 

 phosphate and 3 cwt. of sulphate of ammonia would make a 

 convenient mixture for many purposes, giving about 14 % 

 of soluble phosphates, 24 % of insoluble phosphates and 3 % 

 of nitrogen. 



Potassic Superphosphate. Superphosphate may be 

 mixed with any potash manure or potash manure salts ; 

 these latter have, in the past, usually proved to be the 

 cheapest. Blast furnace dust is too weak for general 

 compounding ; wood ashes contain too little potash and too 

 much carbonate. If the potash manure employed contains 

 much chloride, there is some tendency for the manure to 

 become damp on long standing, but as a rule most of 

 the potash manures help to dry the superphosphate and 

 check retrogression. A very common practice in the past 

 has been to use a potash manure salt containing about 30 % 

 of potash. By using i cwt. of such a potash manure salt 

 and 19 cwt. of superphosphate, a manure containing i| % 

 of potash will be produced ; with 5 cwt. of potash manure 

 salt and 15 cwt. of superphosphate, a compound fertilizer 

 containing 7 J % of potash manure would be produced ; these 

 quantities represent about the limits of ordinary practice. 

 Potassic superphosphate is suitable for grass land on the 

 lighter soils, and on tilled land where nitrogenous constituents 

 are best applied as a top dressing. For mangolds a con- 

 venient application would be 12-15 tons of dung, 5 cwt. 

 of potassic superphosphate containing about 29-39 % of 

 soluble phosphate, and 7 % of potash, leaving the salt and 

 nitrate of soda, say 5 cwt. of the former and 2 cwt. of the 

 latter, to be applied as top dressings in two or three fractions. 

 One reason for such a distribution is that neither the potash nor 

 the phosphate is liable to be washed out of the soil, and both 

 may be applied early ; a second reason is that soluble mineral 

 materials of this kind are required during the germination 



