28 ON SOILS. 



such alkaline matter for their nourishment, the 

 soil will, by the continuous action of the air, 

 &c., in effecting the decomposition of the pri- 

 mitive rocks, again acquire a supply of those 

 alkaline salts to be enabled to perfect a crop. 



The plants which may be termed strictly 

 fallow crops are beans, peas, and buckwheat, 

 as the ashes of these vegetables when analyzed 

 scarcely yield a trace of the alkaline salts, or 

 phosphates; next to these may be classed tur- 

 nips, cabbage, beet and potatoes, which exhaust 

 the soil more than the preceding, but still not 

 to the extent that the white crops, such as oats, 

 barley and wheat, which require such a sup- 

 ply of the alkalis and phosphates for perfecting 

 a crop as usually to exhaust the soil in one 

 season. 



A peculiar case of the rotation of crops oc- 

 curs at Manningford, and other places in the 

 neighbourhood of Pewsey in Wiltshire. There 

 the invariable rotation is wheat and beans, and 

 has been so for fifty years past, experience 

 having taught the agriculturist there, that that 

 systeui is the best, and a knowledge of chemis- 

 try explains how it is, that it is so. 



Wheat absorbs the native phosphate and al- 

 kaline salts, leaving the soil, we will suppose, 

 for the sake of argument, quite exhausted of 

 these necessary ingredients, and leaving of 

 course the excrement which all plants exude 

 from the roots during their growth. The suc- 

 ceeding crop beans, does not require for its per- 

 fection either phosphates or alkaline salts, sim- 

 ply a supply of carbon, and therefore as far as 



