EFFECTS PRODUCED BY PLANTS UPON THE SOIL. 103 



Bcarcely return the seed, and then leave it " to rest" in un- 

 profitaijle pasture. The same mode of cropping was common 

 in many parts of England before the introduction of what 

 has been tenned the Norfolk system, which led the way to the 

 improved management of the soil which at present distin- 

 ^juishes the North of England, and which has been followed 

 and improved in Scotland, and is gradually making its way 

 into this country. In this system the laud is every year 

 made to produce food by a skilful application of the prmciples 

 which have been laid down. 



146. The investigations which have been made within the 

 last two or three years have led to some curious obsei-vations 

 on the composition of the ashes of plants, which, if corroborated 

 by futui'e researches, will be of gi'eat practical importance. 

 It is known to the chemist, that in the mineral kingdom, 

 certain substances are found to replace one another in the 

 composition of minerals ; thus, soda in some minerals takes 

 the place usually occupied by potash, &c. ; and in plants a 

 sunilar curious substitution of one substance for another has 

 been detected. Thus, in the ashes of clover gi'owu upon soils 

 rich in potash and poor in soda, the former of these substances 

 is found in large quantity, while in soils in which soda is 

 abundant and potash deficient, the soda is found to occupy 

 the chief place in the ashes of the plant. The same thing 

 has been discovered in other plants. The ash of the oak, for 

 example, is usually found to contain potash, but on the sea- 

 coast of North America, at Long Island, soda has been found to 

 take its place (Gai'diner). The study of these curious substi- 

 tutions opens an interesting field to the agricultural chemist. 



1 47. The practical rules to be deduced from the foregoing 

 observations are — 



I. That plants which require chiefly the same kind of 

 materials for their support should not be grown in 

 succession. 

 II. That as the efiects which different crops produce upon 

 the fertility of the soil are influenced by the purpose 

 for which they are grown, that plants cultivated for the 

 sake of their seeds^ as wheat, barley, oats, flax, should 

 be made to alternate with those which are cultivated 

 for their roots, foliage^ ov fibre, as turnips, clover, &c, 

 and also hemp and flax when the seeds are not allowed 

 to ripen. 



I 



