AVAILABLE FOOD SUPPLY 283 



cultural soils of the world are now known, only three 

 of the essential plant-foods are likely to be absent, 

 namely, potash, phosphoric acid, and nitrogen; of 

 these, by far the most important is nitrogen. The 

 whole question of maintaining the supply of plant- 

 foods in the soil concerns itself in the main with the 

 supply of these three substances. 



The -persistent fertility of dry-farms 



In recent years, numerous farmers and some 

 investigators have stated that under dry-farm condi- 

 tions the fertility of soils is not impaired by cropping 

 without manuring. This view has been taken be- 

 cause of the well-known fact that in localities where 

 dry-farming has been practiced on the same soils 

 from twenty-five to forty-five years, without the 

 addition of manures, the average crop yield has not 

 only failed to diminish, but in most cases has in- 

 creased. In fact, it is the almost unanimous testi- 

 mony of the oldest dry-farmers of the United States, 

 operating under a rainfall from twelve to twenty 

 inches, that the crop yields have increased as the 

 cultural methods have been perfected. If any 

 adverse effect of the steady removal of plant-foods 

 has occurred, it has been wholly overshadowed by 

 other factors. The older dry-farms in Utah, for 

 instance, which are among the oldest of the country, 

 have never been manured, yet are yielding better 



