In view of this condition and tendency, the fact that dry-farm products are unusually rich in the most valuable nutritive materials is of tremendous importance to the development of dry-farming.  The small average yields of dry-farm crops do not look so small when it is known that they command higher prices per pound in competition with the larger crops of more humid climates.  More elaborate investigations should be undertaken to determine the quality of crops grown in different dry-farm districts.  As far as possible each section, great or small, should confine itself to the growing of a variety of each crop yielding well and possessing the highest nutritive value.  In that manner each section of the great dry-farm territory would soon come to stand for some dependable special quality that would compel a first-class market.  Further, the superior feeding value of dry-farm products should be thoroughly advertised among the consumers in order to create a demand on the markets for a quality valuation.  A few years of such systematic honest work would do much to improve the financial basis of dry-farming.

CHAPER XIV

MAINTAINING THE SOIL FERTILITY

All plants when carefully burned leave a portion of ash, ranging widely in quantity, averaging about 5 per cent, and often exceeding 10 per cent of the dry weight of the plant.  This plant ash represents inorganic substances taken from the soil by the roots.  In addition, the nitrogen of plants, averaging about 2 per cent and often amounting to 4 per cent, which, in burning, passes off in gaseous form, is also usually taken from the soil by the plant roots.  A comparatively large quantity of the plant is, therefore, drawn directly from the soil.  Among the ash ingredients are many which are taken up by the plant simply because they are present in the soil; others, on the other hand, as has been shown by numerous classical investigations, are indispensable to plant growth.  If any one of these indispensable ash ingredients be absent, it is impossible for a plant to mature on such a soil.  In fact, it is pretty well established that, providing the physical conditions and the water supply are satisfactory, the fertility of a soil depends largely upon the amount of available ash ingredients, or plant-food.

A clear distinction must be made between the_ total and available plant-food.  The essential plant-foods often occur in insoluble combinations, valueless to plants; only the plant-foods that are soluble in the soil-water or in the juices of plant roots are of value to plants.  It is true that practically all soils contain all the indispensable plant-foods; it is also true, however, that in most soils they are present, as available plant-foods, in comparatively small quantities.  When crops are removed from the land year after year, without any return being made, it naturally follows that under ordinary conditions the amount of available plant-food is diminished, with a strong probability of a corresponding diminution in crop-producing power.  In fact, the soils of many of the older countries have been permanently injured by continuous cropping, with nothing returned, practiced through centuries.  Even in many of the younger states, continuous cropping to wheat or other crops for a generation or less has resulted in a large decrease in the crop yield.