136 Fertilizers 



There is also a number of plants which require much 

 larger quantities of lime for maximum growth than is 

 naturally contained in soils. Of these plants the legumes, 

 especially alfalfa, form the largest group. 



OCCURRENCE OF LIME 



It is fortunate that lime occurs in great abundance. 

 Reliable estimates show that about one-sixth of the rock 

 mass of the earth's crust is composed of calcium com- 

 pounds. Vast tracts of country are composed of nothing 

 but limestone and a large number of the more common 

 minerals contain high percentages of calcium. Pure lime 

 is insoluble in pure water but is readily soluble in water 

 containing carbonic acid, such as rain water or soil water. 

 Such waters aid greatly in the disintegration of the rock 

 or mineral and carry the lime to the soil. 



In nature, lime exists chiefly as calcium carbonate or 

 carbonate of lime in the forms of limestone, marble and 

 chalk. It also exists in combination with magnesium 

 and other chemical elements. Oyster shells and clam 

 shells are composed almost entirely of calcium carbonate. 

 Gypsum, a different chemical compound referred to else- 

 where, also occurs in nature. 



FORMS ON THE MARKET 



Caustic lime. 



Limestone as it occurs in nature is in the form of a 

 hard rock and as such it is incapable of distribution and 

 likewise incapable of exerting the many functions for 

 which lime is used. Limestone is really a chemical com- 

 bination of calcium oxide and carbonic acid. When 



