May a soil fail to produce crops and yet not be exhausted of 

 plant-food ? 



If there are 13 plant-foods which are positively essential, why 

 do we speak of only 3 of them as plant-food, — of nitrogen, pot- 

 ash, phosphoric acid ? 



Do you know if there is any difference between phosphorus 

 and phosphoric acid ? Write the chemical symbol for each. 



Is there any difference between potassium and potash ? Write 

 chemical symbols for each. 



Write the chemical symbols for calcium and lime. 



Where do phosphorus, potassium and calcium come from,- 

 from the ground or the air ? Are they gases or solids ? 



Where does the oxygen come from, with which they are 

 combined ? 



