160 INTERCHANGE OF CROPS. 



oats, may be cultivated in close succession when 

 proper manure is used. It has also been found, 

 that several of these plants improve the soil, whilst 

 others, and these are the most numerous, impove- 

 rish or exhaust it. Fallow turnips, cabbage, beet, 

 spelt, summer and winter barley, rye, and oats, are 

 considered to belong to the class which impoverish 

 a soil ; whilst by wheat, hops, madder, late turnips, 

 hemp, poppies, teasel, flax, weld, and licorice, it is 

 supposed to be entirely exhausted. 



The excrements of man and animals have been 

 employed from the earliest times for the purpose of 

 increasing the fertility of soils ; and it is com- 

 pletely established by all experience, that they 

 restore certain constituents to the soil, which 

 are removed with the roots, fruit, or grain, or entire 

 plants grown upon it. 



But it has been observed that the crops are not 

 always abundant in proportion to the quantity of 

 manure employed, even although it may have been 

 of the most powerful kind ; that the produce of 

 many plants, for example, diminishes, in spite 

 of the apparent replacement of the substances re- 

 moved from the soil by manure, when they are 

 cultivated on the same field for several years in 

 succession. 



On the other hand it has been remarked, that a 

 field which has become unfitted for a certain kind 

 of plants was not on that account unsuited for 

 another; and upon this observation, a system of 



