THE CONTENTS OF THE ROIL 2? 



of the crops which it is to produce, it is easy to 

 calculate the time when the soil will have lost 

 its power ; but it must be remembered that the 

 materials which the plant removes are consumed, 

 and that the volume of the soil is reduced by 

 that amount. The result is, therefore, that the 

 deeper parts of the soil are brought into requi- 

 sition as fast as the upper parts are consumed ; 

 and these depths will last as long as the earth 

 lasts. 



46. Of some materials, however, the plant 

 uses more freely than of others, in proportion to 

 their abundance in the soil. Therefore the soil 

 may finally lose its productivity, although it is 

 doubtful if it can ever be completely exhausted 

 of plant-food. 



47. Again, the profit in agriculture often lies 

 in making the soil produce more abundantly 

 than it is of itself able to do. That is, even 

 after tillage and every other care have forced 

 the soil to respond to its full ability, it may 

 pay the farmer to buy plant -food in bags in the 

 same way that it may pay him to buy ground 

 feed when fattening sheep. Whether it is ad- 

 visable to buy this plant- food is a matter of 

 business judgment which every farmer must 

 determine for himself, after having considered 

 the three fundamental factors in the problem : 

 the cost of the plant- food (or fertilizer), the 



