144 ROTATION OF CROPS. 



quantity of the organic constituents of the blood (compounds con- 

 taining sulphur and nitrogen) will remove from the soil the 

 greatest amount of inorganic constituents (phosphates). 



The one plant will exhaust a soil of these ingredients, but it 

 may still remain in a good condition for a second kind of plant 

 requiring a smaller quantity of phosphates, and may even be fer- 

 tile for a third kind. 



Hence it happens that the greater development of certain parts 

 of plants, such as the seeds, which contain much more of the or- 

 ganic constituents of the blood than any of the other parts, exhausts 

 and removes from the soil a much greater amount of phosphates 

 than would be done by the culture of herbaceous plants, tubers, 

 or roots, these being proportionally much poorer in the above in- 

 gredients. It is further evident, that two plants growing togethej 

 on the same soil will share its ingredients between them, if they 

 both require in equal periods equal quantities of the same con- 

 stituents. The ingredients taken up into the organism of one of 

 the plants cannot be used by the other. 



If a given space of a soil (in surface and in depth) contains 

 ->nly a sufficient quantity of inorganic ingredients for the perfect 

 development of ten plants, twenty specimens of the same plant, 

 cultivated on this surface, could only obtain half their proper ma- 

 turity ; in such a case, there must be a difference in the number 

 of their leaves, in the strength of their stems, and in the number 

 of their seeds. 



Two plants of the same kind growing in close vicinity must 

 prove prejudicial to each other, if they find in the soil, or in the 

 atmosphere surrounding them, less of the means of nourishment 

 than they require for their perfect development. There is no 

 plant more injurious to wheat than wheat itself, none more hurt- 

 ful to the potatoe than another potatoe. Hence we actually find 

 that the cultivated plants on the borders of a field are much more 

 luxuriant, not only in strength, but in the number and richness 

 of their seeds or tubers, than plants growing in the middle of the 

 •ame field. 



The same results must ensue in exactly a similar manne* 

 when we cultivate on a soil the same plants for successive years, 

 instead of, as in the former case, growing them too closely to- 



