MINERAL DEPOSITS OF FERTILIZERS 43 



As by-products of some other industries, such sources may 

 be useful (see p. 61). 



Limestone Formations. Limestone rocksconsist chiefly 

 of calcium carbonate. Chalk is a white limestone, usually soft, 

 containing sometimes as much as 90 % to 98 % of calcium 

 carbonate. Limestones containing a large amount of silica 

 are too dilute for agricultural purposes. Nearly all lime- 

 stones contain some carbonate of magnesia, those containing 

 a high percentage being called magnesium limestones or 

 dolomites. The limestones belonging to the mountain 

 limestone series are usually comparatively free from magnesia, 

 whereas those of the Permian or magnesium limestone 

 series as a rule contain much magnesia. Even in the 

 Permian series there are large pockets of limestone con- 

 taining little magnesia and 98 % of calcium carbonate. In 

 the absence of an actual analysis of the product of the 

 quarry a knowledge only of the geological formation may be 

 misleading. 



Limestones can be ground in ball mills to a fair degree 

 of fineness (see p. 13). For fertilizers it is quite sufficient 

 to grind the limestone fine enough to be able to pass a sieve 

 containing 50 meshes to the linear inch ; finer grinding 

 is much more expensive with little corresponding advantage. 

 Ground limestone is very suitable for direct application 

 to the land, but the greater part of the limestone quarried is 

 generally converted into lime by burning. The tension of 

 decomposition of calcium carbonate approaches that of 

 atmospheric pressure when the temperature exceeds 800 C., 

 but in practice the temperature for burning lime is markedly 

 higher. When it is of little consequence if the mineral 

 matter of the fuel become mixed with the resulting lime, 

 fuel and limestone are mixed together. Where it is desirable 

 to keep the non-calcareous matter low, efforts are made to 

 keep the two ingredients separate, but for agricultural 

 purposes the presence of a small amount of silica is perfectly 

 harmless. It is, however, essential that the limestone 

 used in the kiln should be of good quality, and contain a high 

 percentage of calcium carbonate, otherwise the resulting 



