68 AMERICAN MANURES. 



tity ; because magnesia, in its caustic state, does 

 not absorb carbonic acid nearly so readily as burnt 

 lime, but remains in the soil in this state for a 

 much longer period ; besides, it will harden into 

 a cement beneath the surface, and lessen the 

 friability and porosity of the soil, and interfere 

 with the growth of the rootlets. 



Magnesia in large quantity might be applied 

 to the soil in the form of ground magnesian 

 limestone ; the magnesia in such state would be 

 gradually liberated by the acids of the soil, and 

 rendered available for the wants of plants as it 

 is needed. 



IRON. 



Iron is a solid that is most widely distributed. 

 There is scarcely a rock or soil, plant or mineral, 

 but contains traces of it, and it scarcely ever be- 

 comes necessary to employ it as a manure. Iron 

 unites with oxygen in two proportions, forming 

 the protoxide and peroxide of iron. These com- 

 binations are gradually effected in a moist at- 

 mosphere. 



Protoxide of Iron, when pure, is presented in 

 the form of a black powder. The scale falling 

 from heated iron is largely composed of it. Its 

 presence in a large amount in a soil is very inju- 

 rious to growing vegetation. Protoxide of iron is 



