50 THE SOILS AND CROPS OF THE FARM. 



yield of wheat ia the United States. On some fer- 

 tile soils in this country ' grain has been profitably 

 grown for more than a half- century without the di- 

 rect application of plant food. On many soils, how- 

 ever, a decrease in the yield of crops is noticed after 

 they have been cultivated for a few years, unless some 

 return of plant food is made. 



The term "exhaustion of soils," although often used, 

 is somewhat misleading. Continuous cropping may 

 reduce the fertility of the soil so that it may not ba 

 profitable to cultivate it longer, but it is not probable 

 any soil could be exhausted, that is, made absolutely 

 barren, by the cultivation and removal of crops. As 

 has been said, the soil is not a finished product. Tha 

 supply of plant food in the soil is not like grain in a 

 bin, continued use from which exhausts the supply. 

 The forces of nature are continually manufacturing 

 available plant food in the soil. More or less of thi? 

 may be washed away, or leach into the sub-soil; some 

 of it passes into the air; man may remove some of it 

 in crops. But all soils, properly so-called, contain 

 plant food enough to support something of a crop. 



It may be said that all soils have a "natural fer- 

 tility." Under like conditions each will produce a 

 crop not greatly varying from year to year. As a 

 consequence of the growth and decay of forest trees 

 or prairie grasses, virgin soils have accumulated 

 a large supply of material for the growth of plants, 

 either available for their use or capable of being made 

 available. This may be called "surplus fertility," "ac- 

 cumulated fertility" or "natural manuring. " Cropping 

 without return of plant food may exhaust this surplus 

 store and reduce the land to its natural fertility. 



