CAUSES OF ITS BENEFICIAL INFLUENCE. 169 



and burnt lime, or even small quantities of ashes 

 which have not been lixiviated, must enable a soil 

 to permit the cultivation of the same plants in a 

 much shorter time. 



A soil lying fallow owes its earlier fertility, in 

 part, to the destruction or conversion into humus 

 of the excrements contained in it, which is effected 

 during the fallow season, at the same time that the 

 land is exposed to a further disintegration. 



In the soils in the neighbourhood of the Rhine 

 and Nile, which contain much potash, and where 

 crops can be obtained in close succession from the 

 same field, the fallowing of the land is superseded 

 by the inundation ; the irrigation of meadows 

 effects the same purpose. It is because the water 

 of rivers and streams contains oxygen in solution, 

 that it effects the most complete and rapid putre- 

 faction of the excrements contained in the soil 

 which it penetrates, and in which it is continually 

 renewed. If it was the water alone which produced 

 this effect, marshy meadows should be the most 

 fertile. 



It follows from what has preceded, that the 

 advantage of the alternation of crops is owing to 

 two causes. 



A fertile, soil ought to afford to a plant all the 

 inorganic bodies indispensable for its existence in 

 sufficient quantity and in such condition as allows 

 their absorption. 



All plants require alkalies, which are contained 



