PHOSPHORUS FERTILIZERS 155 



atmospheres, applied by weight and lever, will show in the 

 course of a few days what retrogression may be expected in 

 any particular sample of manure. It is, however, rare to 

 find a superphosphate that retrogrades more than 1-2 % 

 in the course of twelve months, and it is not easy to obtain 

 consignments of superphosphates which are constant in 

 composition to that extent. 



A Phosphatic Nitrate has been produced in Sweden by 

 acting upon mineral phosphate with the weak nitric acid 

 produced by an electric furnace. To prevent overloading 

 the manure with nitrogen, it is only possible to use so little 

 nitric acid that much of the mineral phosphate remains 

 undissolved. Experiments showed that a large proportion 

 of undissolved phosphates was no disadvantage when used 

 for growing oats. (Compare p. 187.) 



Basic Superphosphate. In the early days of the 

 manufacture of superphosphate, phosphates were often only 

 partially dissolved, and as much as 8-10 % of undissolved 

 phosphate was retained. It was easy to make an excellent 

 dry fertilizer by such methods, but with the endeavour to 

 make the most out of rock phosphates, the amount of insoluble 

 phosphate left behind has been reduced to i % or 2 %. As 

 time went on, superphosphate became damper and more 

 acid, and difficulties arose in its distribution in the ordinary 

 corn drill. Improvements in distributing machines over- 

 came this difficulty to some extent, and greater skill in 

 manufacture, as described above, also tended to produce 

 drier superphosphates; at last an alternative scheme was 

 evolved to treat damp and unsatisfactory superphosphates 

 with a small quantity of lime, so as to obtain them in a 

 drier condition. About 85 parts of superphosphate were 

 thoroughly mixed with 15 parts of good slaked lime, and 

 allowed to remain for at least 24 hours. By these means 

 a fertilizer was obtained which contained very little phos- 

 phoric acid soluble in water ; nevertheless most of the phos- 

 phate present was easily soluble in very weak solutions of 

 citric acid or ammonium citrate. The solubility, as tested by 

 any method, exceeded that of the basic slag. The mechanical 



