b 



THK CONTENTS OK rUh SiJll. 27 



of tlit> crops which it is to produce, it is easy to 

 calcuhite the time when the soil will have lost 

 its power ; but it must he rememljored that th«' 

 materials which the }>laut removes arc consumed, 

 and that the volume of the soil is reduct'd hy 

 that amount. The i-rsult is, then>fore, that the 

 deaper parts of the snil aj'e hnHiirlit into re(|ui- 

 sitioii as fast as the up]»t«i- parts ai'c consumed ; 

 and these depths will last as JouiT ;i^ the earth 

 hists. 



4t>. ( >f some materials, howcvei-, the plant 

 uses more freely than of others, in i»>«)portion to 

 th»'ir ahundance in the soil. Therefore th«' soil 

 may linally lose its prochictivity, althouirh it is 

 doulitfnl if it can ever he conipletfly exhausted 

 of plant-food. 



17. A,<^ain, tiie profit in ai^ricultuic oftm lies 

 in making the soil pi-oduce more ahundantly 

 than it is of itself aide to d(». That is, even 

 after tilla«^e and ev«>rv other care hnve forcecl 

 the soil to respond Ut its full ahility. it may 

 pay tin* farmer to l»uy plant-food in hairs in the 

 same way that it may pay him to l»uy irround 

 feed when fatteninir sheep. Wlwther it is ad- 

 visable to buy this plant -foo<l is a matter of 

 business judpinent which every farmer nuist 

 determine for himself, after having considered 

 the three fun<lamental t'aetors in the pmhlem : 

 the cost of the plant -t' I (or fertilizer), the 



