Practice and experiment have shown that such diminishing fertility may be retarded or wholly avoided, first, by so working or cultivating the soil as to set free much of the insoluble plant-food and, secondly, by returning to the soil all or part of the plant-food taken away.  The recent development of the commercial fertilizer industry is a response to this truth.  It may be said that, so far as the agricultural 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 conditions the fertility of soils is not impaired by cropping without manuring.  This view has been taken because 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 increased.  In fact, it is the almost unanimous testimony 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 to-day than they did a generation ago.  Strangely enough, this is not true of the irrigated farms, operating under like soil and climatic conditions.  This behavior of crop production under dry-farm conditions has led to the belief that the question of soil fertility is not an important one to dry-farmers.  Nevertheless, if our present theories of plant nutrition are correct, it is also true that, if continuous cropping is practiced on our dry-farm soils without some form of manuring, the time must come when the productive power of the soils will be injured and the only recourse of the farmer will be to return to the soils some of the plant-food taken from it.

The view that soil fertility is not diminished by dry-farming appears at first sight to be strengthened by the results obtained by investigators who have made determinations of the actual plant-food in soils that have long been dry-farmed.  The sparsely settled condition of the dry-farm territory furnishes as yet an excellent opportunity to compare virgin and dry-farmed lands and which frequently may be found side by side in even the older dry-farm sections.  Stewart found that Utah dry-farm soils, cultivated for fifteen to forty years and never manured, were in many cases