vn THE NITROGEN REQUIRED BY PLANTS 63 



We have also learnt that when we remove from the 

 land the crops as fast as they grow up, we are taking 

 away from the soil the store of mineral and vegetable 

 food that the plant requires, and the soil becomes 

 temporarily exhausted, and we have seen how this 

 exhaustion of the soil can be remedied. 



We have still much to learn about the nature of 

 the soil and the changes which take place in the 

 plants, for which purpose we must study very 

 carefully the science of chemistry and then more 

 advanced works on agricultural chemistry; for the 

 present we must be content that we have gained 

 some little insight into the way plants are fed. 



