PHOSPHORUS FERTILIZERS 127 



slag for that land alone, and to use acid phosphatic manures 

 for the remainder of the farm lands. There will remain to 

 some degree a little choice between the use of finely ground 

 rock phosphates and superphosphate, but it is not probable 

 that undissolved rock phosphates can be used extensively, 

 although there is much to be said for encouraging their use 

 on certain particular classes of grazing land or permanent 

 hay. Further, as shown on p. 187, there is much scope for 

 mixtures of superphosphate and rock phosphate. 



Superphosphate. The attention of Lawes was drawn 

 as early as 1837 to the problem of the better utilization of 

 phosphatic manures. At that time machinery for grinding 

 was far less efficient than it is now, so he fell back upon the 

 use of a chemical means for improving distribution. It 

 is quite possible, had he had at his disposal suitable ball 

 mills, that the superphosphate industry might have waited 

 many years before it became developed, but as things 

 happened, the advantage of dissolving rock phosphates 

 in sulphuric acid was very striking indeed. The new 

 superphosphate so obtained was employed on a large scale 

 on crops at Rothamsted in 1841, and in 1842 the 

 results were so satisfactory that L,awes took out a patent 

 for the manufacture of superphosphate. The treatment of 

 bones with sulphuric acid had been practised before Lawes' 

 patent, the novelty at the time consisting in the treatment of 

 mineral phosphates. It is often forgotten, when alluding 

 to the work of l,awes and Gilbert, that I^awes was a manu- 

 facturer of artificial manures quite as much as a farmer, and 

 was probably as much financially interested in the business of 

 manufacturing artificial manures as he was in the business of 

 growing crops. When very finely ground, these mineral 

 phosphates have much value, but by treatment with sulphuric 

 acid their fertilizing efficiency can be enormously increased. 

 The manufacture of superphosphate in Great Britain probably 

 amounts to nearly a million tons per annum, and the total 

 production of the world is about eleven times that quantity. 



The first step in the production of superphosphate 

 consists in the manufacture of sulphuric acid; although 



