110 



OF AGRICULTURE 



virgin soils are so productive that farmers nearly always 

 make the mistake of thinking that they will always remain 

 so. But the constant tillage exhausts the humus supply, 



and our virgin soils be- 

 come less and less pro- 

 ductive. The change is 

 so gradual and is so ob- 

 scured by the weather 

 variations from year to 

 year that the real state 

 of affairs is often not 



Fie. 49. Corn crop on a farm that has raised 

 * little live stock for fifty years 



;^f* 



fc*^3HMB 



realized until the soil is 

 so poor that it does 

 not pay to farm it. Sooner or later every farmer must 

 give attention to means of maintaining the productivity 

 of the land, no matter how rich the original soil may be. 

 Thirty to sixty years of grain farming usually exhausts 

 a rich virgin soil to such an extent that grain farming 

 no longer pays. It then 

 becomes necessary to 

 raise stock and use ma- 

 nure or to plow under 

 green-manure. Some- 

 times commercial fer- 

 tilizers are resorted to 

 and these may pay for 

 a few years, but sooner 

 or later some provision 

 for renewing the humus supply must be made, or the 

 field must be temporarily abandoned to allow nature to 

 renew the supply by growing weeds. Many fields in the 



Fi. 60. Corn crop on a dairy farm near 

 Fig. 49 



