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 
