ACTION OF LIME. 121 



The same chemical actions as those now described proceed in 

 our arable land, and it is to accelerate and increase these that we 

 employ the mechanical operations of culture. We renew the 

 surface of the soil, and endeavor to make every particle of it 

 accessible to the action of carbonic acid and of oxygen. Thus 

 we procure a new provision of soluble mineral substances, which 

 are indispensable for the nourishment and luxuriance of a new 

 generation of plants. 



All cultivated plants require alkalies and alkaline earths, 

 although each of them may use different proportions of the one 

 or of the other ; the cereals do not flourish in a soil deficient in 

 silica in a soluble state. 



Silicates, as they occur in nature, differ very materially in 

 their tendency to suffer disintegration, and in the resistance 

 which they offer to the action of atmospheric agents. The 

 granite of Corsica and the felspar of Carlsbad crumble into dust 

 in a space of time during which the polished granite of the Berg- 

 strasse does not even lose its lustre. 



There are certain kinds of soil so rich in silicates prone to 

 disintegration, that every year, or every two years, a quantity of 

 silicate of potash is rendered fit for assimilation sufficient for the 

 formation of the leaves and stems of a whole crop of wheat. In 

 Hungary there are large districts of land, on which, since the 

 memory of man, corn and tobacco have been cultivated in alter- 

 nate years, without the restoration of the mineral ingredients 

 carried away in the corn and in the straw. There are other 

 fields, on the contrary, which do not yield sufficient silicate of 

 potash until after two, threa, or more years. 



Fallow, in its most extended sense, means that period of cul- 

 ture during which a soil is exposed to the action of the weather, 

 for the purpose of enriching it in certain soluble ingredients. Iu 

 a more confined sense, the time of fallow may be limited to the 

 interval inthe cultivation of cereal plants ; for a magazine of 

 soluble silicates and of alkalies is an essential condition to the 

 existence of such plants. The cultivation of potatoes or of tur- 

 nips during the interval will not impair the fertility of the field 

 for the cereals which are to succeed (supposing the supply of 



