SOILS AND FERTILIZERS 



101 



careful tillage, by rotation of crops, and by utilizing the 

 straw and manure of the farm to postpone the exhaus- 

 tion of the plant food elements of the soil. If, how- 

 ever, crops are sold year by year from the farm, either 

 in the form of grain, hay, or live stock, the soil from 

 which these crops come must become less productive. 



What plant food is removed from the soil in crops 

 must be returned to the soil in some form, if the fertil- 

 ity is to be maintained. It is the province of the farmer 

 to secure from the soil as large a crop as possible, sell 

 it for as high a price as he can get, and put back into 

 the soil the fertility removed from it at as small an ex- 

 penditure as possible. 



Fertility removed by Different Crops. As will 

 be seen from the table in the Appendix, different crops 



800 Lbs, OF TOBACCO 



FIG. 37. Showing the Pounds of Plant Food removed by 800 Pounds of 

 Tobacco. 



remove different proportions of the important plant 

 foods containing nitrogen, phosphorus, and potassium. 



