108 



As a palliative, the system called " rotation of 

 crops" then came into practice, the soil refused 

 to grow the same description of crop year after year 

 without receiving back the elements of plant-food 

 it had been deprived of; it was temporarily 

 exhausted of available substances constituting the 

 inorganic plant-food ; it required a period of rest 

 to be able to recover and reproduce them in 

 an assimilable form from the material stored up in 

 it, and nature effected this by the action of 

 moisture, heat, and atmospheric air. In the mean- 

 time, different crops were grown which derived their 

 plant-food from the soil of a lower stratum, inacces- 

 sible to the roots of the first description of crop, or 

 which required somewhat different plant-food ; but 

 finally it was found necessary to allow the land to 

 remain idle altogether for a season, after certain 

 intervals, in order to restore the capability of the 

 soil of yielding remunerative harvests. This 

 system has been called " fallowing. 7 ' 



Notwithstanding all these expedients, however, 

 the yield of the crops became perceptibly smaller, and 

 the farmer was compelled to resort to other means 

 to avert the impending ruin, and the importation 

 of manure for the use of farms, as an equivalent for 

 the crops and other products which had been ex- 

 ported from them, then first came into vogue ; here 

 the farmer had at last arrived at the only rational 

 principle on which farming for exportation can be 



