If8 niOSPIIATIC MANURES [chap. 



All these phosphate deposits are now yielding to 

 the competition of the <;reat deposits of phosjihate 

 rock which have been discovered in Northern Africa 

 and are now bein^ exported in immense quantities 

 from Al}^cria and Tunis. The phosphate bed appears 

 to stretch right across the continent, but Morocco has, 

 naturally, not been explored, while the Egyptian rocks 

 as yet examined are hardly rich enough in phosphoric 

 acid for export, though immense beds exist containing 

 40 to 50 per cent, of tricalcium phosphate. The most 

 important of the phosphate mines in North Africa occur 

 in the province of Constantine in the district of Tebessa, 

 from whence they extend into Tunis, near Gafsa. The 

 rock is generally at the base of the Kocene .system, and 

 occurs in strata that may be 2i or 3 metres thick and 

 contain as much as 60 per cent of calcium phosphate, 

 which may be raised to 70 per cent, by picking 

 over. These African phosphates contain but little iron 

 and alumina, an<i are rapidly becoming the chief 

 material for the manufacture of superphosphate in this 

 country. 



The mineral phosphates have been but little 

 employed directly as manures, though there is plenty 

 of evidence that when they are really finely ground 

 they are effective enough on soils retaining plenty of 

 water, and particularly on those of a peaty nature. 

 Recent experiments also indicate that such ground 

 mineral phosphates are most available when used 

 with ammonium sulphate, which, as already explained, 

 acts as a physiologically acid manure and helps to 

 bring the phosphoric acid into solution. In the main, 

 however, the mineral phosphates are used for the 

 manufacture of superphosphate, practically the only 

 manure which contains phosphoric acid at all readily 

 soluble in water. 



