Manures and Chemical Applications. 241 



Modifying Agents. — ^We have said that the part 

 played by these agents consists chiefly in modifying the 

 original composition of the soil, so that it may the more 

 readily lend itself to the full action of manures, and to 

 the profitable influence of atmospheric agents. The 

 application of modifying agents, so useful for herbaceous 

 plants, are no less so for those or a woody nature, and 

 more particularly for the vine. The principal materi- 

 als of this class are the following : 



Chalks. — The prolonged cultivation of the vine, in 

 clayey soils, results in developing acid principles hurt- 

 ful to vegetation. The application of chalk modifies 

 this acidity. Moreover, if the ground is rather stifF, 

 chalk has the further effect of loosening the soil and 

 making it more pervious to air and water. 



Chalky marls are usually employed for this purpose. 

 They should be friable just in proportion as the soil to 

 be modified happens to be stiff and clayey. These 

 marls should be used in the proportion of about sixty- 

 six bushels to the acre. The application of marls is 

 also very effective in flinty soils, but the proportion 

 should only be twenty bushels to the acre. 



The marl is spread over the ground at the beginning 

 of winter. It breaks up under the influence of frosts. 

 It is then spread out as evenly as pftssible, and finally 

 buried, at the first winter plowing. This process may 

 be repeated every twelve or fifteen years. 



[The marls of some parts of our country are a material of 

 great value to agriculture, and they are becoming more and 

 more highly appreciated by the farmers. Those found in New 

 Jersey and other places in the cretaceous formation, and known 

 as the green-sand marls, are rich in potash, yielding from ten 

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