168 THE INTERCHANGE OF CROPS. 



It will take place very quickly in a calcareous soil ; 

 for the power of organic excrements to attract oxy- 

 gen and to putrify, is increased by contact with the 

 alkaline constituents, and by the general porous 

 nature of such kinds of soil, which freely permit the 

 access of air. But it requires a longer time in 

 heavy soils consisting of loam or clay. 



The same plants can be cultivated with advan- 

 tage on one soil after the second year., but in others 

 not 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 

 completed in the one, in the second year ; in the 

 others not until the ninth. 



In some neighbourhoods, 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 advantage, 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 pro- 

 duced. 



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

 the excrements of which, in argillaceous soils, 

 require the longest time for their conversion into 

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



