PREFACE ix 



munity use no less than 890,000 tons per annum, equivalent to 

 33,820,000 units of tricalcicphosphate. On the other hand the 1920 

 output of British steel yielded about 560,000 tons of slag of 15| per 

 cent, or higher content of phosphate, equivalent to 13,400,000 units 

 of tricalcicphosphate. There is therefore a considerable gap between 

 the farmers' potential demand and the visible supply. The difficult 

 problems associated therewith are being fully and sympathetically 

 studied by agriculturists and steel-making experts and no doubt 

 various solutions will be devised. One obvious possibility is to use 

 ground mineral phosphates to stiffen out the supplies, and here 

 Dr Robertson's experiments will prove helpful. 



Dr Robertson has not confined himself to the practical demonstra- 

 tion of increased yields: he has gone further and endeavoured to 

 ascertain why the increases have been obtained, thus giving the 

 monograph a scientific as well as an empirical interest. He examines 

 the change in herbage and he shows that the physical properties of 

 the soil and the bacterial actions in the soil are much influenced by 

 the phosphate in the slag, thus throwing important light on the view 

 now commonly held by experts that poor grassland should not be 

 ploughed out till after it has been improved by slag. 



The monograph contains a store of information about the new slags 

 and is a model of thorough and systematic investigation. I have 

 personally inspected the plots on several occasions and have seen 

 much of the experimental work. It deserves close study by all who 

 are interested. 



E. J. R. 



January, 1922. 



