SCIENTIFIC AGRICULTURE. 209 



to be repeated once in 10 or 12 years. With regard to the 

 quantity of marl which should be used to an acre, we must be 

 governed by the same rational considerations as the use of all 

 other manures ; viz., it should be applied where it is required, 

 and in quantity equal to the demand of the soil. The opin- 

 ions of practical men vary greatly on this subject : according 

 to Johnston, ten or fifteen, to one hundred and twenty tons 

 are used to an acre ; while Boussingault says, " allowing the 

 broadest margin, and judging from the composition of the ashes 

 of the plants of ordinary crops, we can see that the quantity 

 of three and a half bushels of marl of the usual composition 

 per acre, which is assumed as the average quantity to be laid 

 on, is vastly more than can be absolutely necessary." 



This discrepancy has arisen partly from the extravagant 

 notions about the virtues of marl, and partly from the nature 

 of the marl and the soils to which it has been applied by dif- 

 ferent experimenters. 



Chalk is much used as a fertilizer in some parts of Europe 

 where it is cheap and abundant ; but, from its scarcity and 

 price, it can never be expedient to use it in this country while 

 we have such an abundance of lime in various other forms. 

 When used, it is subject to nearly the same laws as lime and 

 marl. Its composition varies ; some specimens contain more 

 phosphate of lime, magnesia and silicates, than others. Ehren- 

 berg has made the remarkable discovery, that chalk to a con- 

 siderable extent, is composed of the shells or skeletons of ma- 

 rine microscopic animals. 



Lime. The chemical and physical properties of lime have 

 already been described, and it remains for us to examine briefly 

 the principles of its adaptation to the soil as a fertilizer. Much 

 discussion has been had, and many long essays written on this 

 subject; but no chemist claims for this substance any excep- 

 tion to general chemical laws, or attributes to it any action 

 more specific than that of any other manure. There is no 



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