By these circulation processes the supply of carbon dioxid in the atmosphere is renewed and maintained without any special effort on the part of man.  Hydrogen is one of the elements of which water is composed.  Water is taken into the plant through the roots, carried through the stems to the leaves, and there, under the influence of chlorophyll, sunlight and the life principle, the carbon, oxygen and hydrogen are made to unite into some of the most important plant compounds, such as the sugars, which are later transformed into starch and fiber.

Though these three elements constitute the larger part of the mature agricultural plant they are no more necessary for plant growth than the seven which are supplied by the soil.  Iron is one of the essential elements of plant food; but the amount required by plants is so small and the amount contained in the soil is so large that soils have never been known to become deficient in iron.  Though sulfur is found in plants in very appreciable amounts and is known to be essential to plant growth, it is evident that plants do not need so much sulfur as they often contain, some of it being taken up and merely tolerated, as is the case with all of the sodium and silicon found in plants, neither of these being required for normal growth, although commonly found in plants in very considerable amounts.  The supply of sulfur in normal soils is not large; but, with the combustion and decay of organic materials—­coal, wood, grass, leaves, and so forth—­sulfur passes into the air and is brought back to the soil dissolved in rain or absorbed by direct contact of soil and air.  Thus under normal conditions the supply of sulfur naturally provided is ample to meet the needs of the staple farm crops, although there are some plants, such as cabbage, for example, which may possibly be benefited by fertilizing with sulfur.

But there are five other essential elements of plant food, and these require special consideration in connection with permanent soil fertility.  They are potassium, magnesium, calcium, phosphorus and nitrogen.  There are also five important points to be kept in mind in relation to each of these elements:  (1) the soil’s supply, (2) the crop requirements, (3) the loss by leaching, (4) the methods of liberation, and (5) the means of renewal.

The neglect of one or more of these important points in relation to one or more of these five elements has reduced the fertility of most cultivated soils in the United States, has greatly impoverished the older farm lands, and has brought agricultural abandonment to millions of acres in the original thirteen states.  On the other hand, intelligent attention to these same factors will bring restoration and high productive power to such lands.

England’s Best Lesson in Farming

Where these five elements were supplied regularly to land on the Rothamsted Experiment Station the average yield of wheat for the thirty years, 1852 to 1881, was 35.9 bushels an acre, while 13.6 was the average yield of similar unfertilized land; and during the next thirty years—­1882 to 1911—­the corresponding average yields were 38 bushels an acre on the fertilized land, and 11.7 bushels where no plant food was applied.  These statements are not mere opinions, but determined facts whose accuracy stands unquestioned.