226 



THE SYSTEM OF FAKM-YAKD MANURING. 



more back by the increased produce of farm-yard ma- 

 nure, still the ultimate results remain the same. 



In the system of cultivating by rotation of crops, it 

 is found that, for a long time, the arable soil grows with 

 each period of rotation very much richer than it is by 

 nature, in potash as well as in lime, magnesia (the prin- 

 cipal constituents of clover and turnips), and in silicic 

 acid. 



These substances are the principal conditions for the 

 formation of roots and leaves ; their accumulation in 

 the soil tends to make the ground rank and prone to 

 grow weeds,* as the farmer says, an evil which arises 

 as a necessary consequence from cultivation by the sys- 

 tem of farm-yard manuring, and which can only be 

 met, as he thinks, by a rotation of crops. 



It is generally supposed that the best remedy is the 

 hoe ; but though mechanical application may retard the 

 developement of weeds for a time, it cannot effectually 

 prevent them. The hoe has some share in removing 

 them, but not all. 



* The most noxious of these weeds are the wild radish (Raphanus 

 raphanistrum\ the corn cockle (Agrostemma cithago), the corn-flower or 

 blue-bottle (Centaurea cyanus), the German camomile (Matricaria chamo- 

 milla), and the corn camomile (Anthemis arvensis). All these plants con- 

 tain, in their ash, as much potash as is found in clover, and 7 to 18 per 

 cent, of chloride of potassium, a salt which forms one of the principal 

 constituents of the urine of animals, and which is brought to the field in 

 the farm-yard manure. 



(RULING, ' Annal. d. Chem. und Pharm.' vol. Ivi. p. 122.) 



