X.] VALUE OF ROTATION OF CROPS 299 



planation upon the ordinary theories of plant nutrition 

 which have been advanced by Whitney and his co- 

 workers. The soil however is such a complex medium 

 — the seat of so many and diverse interactions, chemical, 

 physical, and biological — and is so unsusceptible of 

 synthetic reproduction from known materials, that 

 experimental work of a crucial character becomes 

 extremely difficult and above all requires to be inter- 

 preted with extreme caution and conservatism 



