168 THE ALTERNATION OF CROPS. 



until the fifth or ninth, merely on account of the 

 change and destruction of the excrements, which 

 have an injurious influence on the plants being com- 

 pleted in the one, in the second year; in the others, 

 not until the ninth. 



In some neighborhoods clover will not thrive till 

 the sixth year, in others not till the twelfth ; flax in 

 the second or third year. All this depends on the 

 chemical nature of the soil, for it has been found by 

 experience, that in those districts where the intervals 

 at which the same plants can be cultivated with ad- 

 vantage are very long, the time cannot be shortened 

 even by the use of the most powerful manures. The 

 destruction of the peculiar excrements of one crop 

 must have taken place before a new crop can be 

 produced. 



Flax, peas, clover, and even potatoes, are plants 

 the excrements of which, in argillaceous soils, re- 

 quire the longest time for their conversion into 

 humus ; but it is evident, that the use of alkalies 

 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 efl*ected 

 during the fallow season, at the same time that the 

 land is exposed to a further disintegration. 



In the soils in the neighborhood 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 field, the fallowing of the land is superseded by the 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 putrefaction of the ex- 

 crements contained in the soil which it pene-trates, 

 and in which it is continually renewed. If it was 

 the water alone which produced this effect, marshy 



