210 THE LURE OF THE LAND 



the course of fifteen or twenty years of such treatment 

 it is found that the annual average production of that 

 field is reduced to about thirteen bushels per acre, and 

 it is not possible by successive crops to reduce it any 

 further. In other words, nature holds back from the 

 avaricious hand of man all stores of plant food beyond 

 that necessary to produce a minimum crop. Thus na- 

 ture safeguards the future from the rapacity of the 

 present. 



This experimental demonstration at Eothamsted has 

 been verified by the natural results of American agri- 

 culture. The virgin fields of our country, suited to 

 the production of wheat, yielded at first from twenty- 

 five to thirty-five bushels of wheat per acre. ISTow that 

 yield has been cut down to about thirteen or fourteen 

 bushels per acre, and beyond this, apart from seasonal 

 variations, we cannot go. Thus it is evident that up to 

 a certain minimum limit the fertility of the soil is car- 

 ried off by successive crops when nothing is returned. 



This latter condition is typified by many of the 

 fields of this country at the present time. The vast 

 areas that produce wheat and corn have never received 

 any fertilizer up to within the past quarter of a century, 

 and it may be said, even to-day, that in so far as the one 

 hundred millions of acres devoted to corn culture are 

 concerned, a very small percentage of this area is ever 

 fertilized either from the stable or from the factory. 

 This, then, is the road of exit which a large part of the 

 soil fertility has taken. 



LOSS BY DRAINAGE AND EROSION. 



A second factor in the loss of soil fertility is seen in 

 the drainage waters of the country. When the surface 

 of the soil was covered by forests and by prairie, the 



