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 tlie air. 



2. ^Yhat do you mean lohen 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 

 plantfood f 



Yes ; the plant-food must not only be present but 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 j)i'oduce a good crop, even though it contains an abundance of 

 plant-food. 



4. If there are \Z plantfoods which are p>ositively essential^ why 

 do we commonly sp)eak of only 3 of them as plantfoods — of nitro- 

 gen^ jyotash^ 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 know if there is any difference between phosphorus 

 and phosphoric 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, P20g. 



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

 chemical symbols for each. 



Potash is potassium plus oxygen, combined in a certain definite 

 proportion. K stands for potassium ; K2O for potash. 



7. W^'ite the chemical syndjols for calcium and lime. 

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



8. Where do p)hospho7'us^ potassium and calcium come from — 

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



