208 THE SYSTEM OF FAKM-YAKD MANUEING. 



the same supply of farm-yard manure will not produce, 

 by any means, equal crops of corn, because these must 

 bear a relation to the corn -constituents supplied in the 

 manure. Of these, both fields received the same 

 amount in the same quantity of manure ; but as the 

 one field, of itself, was richer in corn-constituents than 

 the other, the poorer of the two must receive much 

 more manure to make it produce as large crops as the 

 other. 



A comparatively small quantity of superphosphate 

 will, on a field of the kind, serve to increase the produce 

 to a much greater extent, than the most liberal use of 

 farm-yard manure. 



Upon a field deficient in potash farm-yard manure 

 acts by the potash contained in it ; upon a soil poor in 

 magnesia or lime, by its magnesia or lime ; upon one 

 poor in silicic acid, by the straw in it ; upon land poor 

 in chloride or iron, by the chloride of sodium, chloride 

 of potassium, or iron contained therein. 



This fact accounts for the high favour in which 

 farm-yard manure is held by practical farmers. As the 

 dung of the farm-yard contains, under all circumstances, 

 a certain quantity of each of the mineral constituents 

 withdrawn from the soil by the crops grown on it, its 

 action is universally beneficial. It never fails to pro- 

 duce the desired effect, and thus spares the practical 

 man the trouble of devising more suitable and equally 

 efficacious means for keeping up the fertility of his 

 fields, with a less profuse expenditure of money and 

 labour, or of raising his land, without additional outlay, 

 to the highest attainable degree of fertility compatible 

 with its composition. 



It is well-known in practice, that the produce of 

 many fields may be increased by manuring with guano, 

 bone-dust, rage-cake, and other substances containing 

 only certain constituents of farm-yard manure ;. and 

 their operation is explained, in effect, by the doctrine 

 of mimmum, which I have just laid down. 



But as the practical farmer is not acquainted with 

 the law which regulates the operation of these manur- 



