118 Bulletin 174. 



no. 3, fertility of the soil : what it is. 



1. Do plants obtain all their food from the soil f 

 A part comes from the soil and a part from the air. 



2. What do you mean when you say that soil is exhausted, — that 

 it has no more plant food in it, or merely that it fails to produce 

 crops f 



When a soil merely fails to produce a crop, it is usually said to be 

 exhausted, regardless of the amount of plant-food which it may 

 contain. 



3. May a soil fail to produce crops and yet not he exhausted of 

 plant food f 



Yes ; tlie plant-food must not only be present l)ut it must be in 

 such a form that the plants can use it. The physical condition of 

 the soil also has much to do with the size of the crop. A soil which 

 is hard and lumpy, containing an insufficient amount of humus, will 

 not produce a good crop, even though it contains an abundance of 

 plant-food. 



4. If there are 1^ plantfoods lohich are positively essential, why 

 do we commonly speah of only 3 of them as plant foods — of nitro- 

 gen, potash, phosphoric acid f 



All the other plant-foods are sufficiently abundant in an available 

 form in most soils, so that they do not have to be considered in 

 maintaining .the fertility of the land. 



5. Do you hiow if there is any difference between pjhosphorus 

 and 2)hosphoric acid? Write the chemical symbol for each. 



Phosphoric acid is a certain amount of phosphorus plus a certain 

 quantity of oxygen. The symbol for phosphorus is P ; for phos- 

 phoric acid, PsOg. 



6. Is there any difference between potassium and potash f Write 

 chemical symbols for each. 



Potash is potassium plus oxygen, combined in a certain definite 

 proportion. K stands for potassium ; KgO for potash. 



Y. Write the chemical symbols for calcium and lime. 



Ca is the symbol for calcium ; CaO stands for lime. 



8. Where do phosphorus, potassium and calcium come from — 

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



