8 INTRODUCTION 



The total production of basic slag, including in that term slags con- 

 taining from 1 1 % tricalcium phosphate up wards, amounted to 680,000 

 tons in 1920, but, if all slags below 22 % are excluded, to only 258,600. 

 Of the totals a very large proportion was fluorspar slag. In 1919 

 there was a consumption of at least 560,000 tons and the demand is 

 steadily increasing 1 . Whilst therefore the steel industry may continue 

 to be a valuable source of insoluble phosphates for agricultural pur- 

 poses, it is becoming increasingly evident that the supply can no 

 longer keep pace with the demand and the agriculturist must turn 

 to other sources of supply. 



ROCK OR MINERAL PHOSPHATES 



The increasing demand for basic phosphates can most readily be 

 met by increasing the output of the apparently inexhaustible stores 

 of rock or mineral phosphates and utilising these materials for direct 

 application. Unfortunately there are no extensive deposits in Great 

 Britain 2 and there are not many sources of supply within the British 

 Empire. (Collins in Chemical Fertilisers gives a map showing the 

 distribution of the chief deposits of rock phosphates.) 



Broadly speaking the deposits may be divided into two types 

 the softer and woollier North African phosphate such as Gafsa, 

 Egyptian and Algerian phosphates and the harder North American 

 and Island phosphates such as Florida pebble, Carolina, Nauru Island, 

 and Ocean Island phosphates. 



The deposits in the majority of cases are close to the surface and 

 can be worked, and, in the case of the Island phosphates, transported 

 to the shore and shipped at a comparatively low cost. Plates I and II. 



The North African phosphates are more soluble by the Wagner 

 test than the harder American phosphates (20). They apparently con- 

 tain more calcium carbonate and less calcium fluoride combined in 

 the phosphate compound than is the case with the American phos- 

 phates (21). It may therefore be just as important to distinguish 

 between these two types of rock phosphates as it is to distinguish 

 between open hearth fluorspar basic slag and the open hearth basic 

 slag produced without the use of fluorspar. 



Rock phosphates have the great advantage of a high phosphatic 

 content, ranging in the case of the North African, the Island phos- 



1 Middleton estimates our requirements of basic slag at 891,000 tons per annum. 



2 The deposits of coprolites in Cambridge and Suffolk can no longer be worked 

 economically. 



