214 THE CULTURE OF FARM CROPS. 



which occur in the character of many mineral compounds 

 in the soil, by which these are fitted to act as nutriment for 

 plants. So that, on the whole, mineral manures or fertiliz- 

 ers are of quite as great importance to the farmer as the 

 other classes of manures, and should be equally well under- 

 stood. 



The most important of the mineral manures are lime; 

 gypsum; wood ashes; salt; phosphate of lime; potash; and 

 guano. In these are contained every inorganic element of 

 plant substance that is ever necessary for the growth of 

 crops. Lime is the most important of them, not because it 

 is any more requisite or indispensable than the others, but 

 because of its peculiar effects upon the soil, and the large 

 proportion of it which enters into the structure of vegetable 

 tissue. 



LIME, as has been explained in the description of the me- 

 tallic element calcium, is never found naturally excepting 

 in a state of combination, and mostly as a carbonate, con- 

 sisting of 43.7 per cent of carbonic acid with 56.3 per cent, 

 of its own substance. 



Carbonate of lime is one of the most common of the rocks 

 and is best known in the form of marble. It is frequently 

 combined with carbonate of magnesia, which consists of 

 51.7 per cent, of carbonic acid and 48.3 per cent, of mag- 

 nesia. The carbonate of magnesia is combined in varying 

 proportions with the carbonate of lime, and sometimes some 

 alumina and phosphoric acid are mingled with these. When 

 the magnesia and alumina are in excess, the lime has the 

 property of setting hard under water and is known as hy- 

 draulic or water lime. This class of lime is useless, if not 

 injurious, for agricultural purposes. Lime is procured by 

 calcination, in kilns, of the limestone; in which process the 

 carbonic acid is driven off and the caustic or quick lime re- 

 mains. 2000 Ibs. of limestone yields 1126 Ibs. of quick lime,, 

 and increases about one-third in bulk. Its affinity for wa- 

 ter and carbonic acid is very active; in a moist atmosphere 

 or by mixture, it absorbs about one-third its weight of wa- 

 ter, (9 Ibs. for every 28) swells to three times its original 



