216 THE CULTURE OF FARM CROPS. 



elements and preparing them for plant food. Its action in 

 freshly manured soil, to which it is usually applied, is there- 

 fore of the greatest advantage to the crop ; this action going 

 on slowly in the soil and providing a continuous supply of 

 nutriment for the crops. 



Third. It exerts a peculiar action as a nitrifying agent 

 in the soil by which nitric acid is produced, and by its com- 

 bination with this acid as a nitrate, by which the acid is 

 fixed and retained in the soil, to be afterwards taken up by 

 the potash or other alkaline substances, and finally absorbed 

 as food for the crops; and thus become a most important 

 source of the nitrogen found in the plants. 



Fourth. It exerts a strong solvent action upon the sili- 

 cates in the soil, by which inert and insoluble combinations 

 of silica with potash, soda, magnesia, &c., are broken up; 

 and these foods for plants are made available for the crops. 



Fifth. Its strongly alkaline properties neutralize what- 

 ever injurious acids may exist in the soil; and these are ren- 

 dered innoxious, or in many cases beneficial to the growth 

 of crops. 



Sixth. It has a most beneficial mechanical action upon 

 all soils; loosening, and mellowing, and warming, heavy 

 cold clays; and compacting and making more retentive of 

 moisture light sands; and converting cold peaty soils into 

 warm vegetable mold and fitting them for arable purposes. 

 In addition to these most useful properties, lime has a direct 

 beneficial action upon the growth of wheat and other grains, 

 but especially upon grass and clover; the latter crop grow- 

 ing most luxuriantly whenever lime has been applied to the 

 land. 



MARL, is an impure form of carbonate of lime. It is 

 frequently found underlying swamps, or in low grounds 

 which are the dried up beds of former lakes or ponds in 

 which minute shell fish or more correctly molluscs have 

 existed. The shells of thousands of generations of these 

 creatures have been collected at the bottom of the ponds; 

 and have formed beds of considerable depth; leaving amass 

 of white pulverulent clayey matter intermingled with shells 



