Excerpt from Report of the Chief of the Bureau of Chemistry and Soils, 1937

The production of agricultural materials through the medium of plant growth depends on chemical processes, energized by sunlight, in which the raw ma terials are carbon dioxide; oxygen, and nitrogen of; the air and numerous soil constituents including water, inorganic compounds derived from minerals, and organic compounds j formed through the action of -'micro-organisms on the products of former plant fand animal life. Whether or not these chemical processes go on depends not only on the availability of energy, but also on the availability of raw materials. Since the gases of the atmosphere and sunlight, in most places and at certain seasons, are always sufficiently abundant, it is the availability of raw materials from the soil which determines the course and efficiency of nature's chemical processes that result in agricultural plant and animal products. One of'the most important of these raw materials is water, the availability of which in suitable quantity depends primarily onr the physical and chemical properties of the soil colloids which make it possible for soils, in varying degree, to retain water for fairly long periods after wetting. The properties of their colloids also make it possible for some soils to retain, in spite of frequent wetting with excessive quantities of water, inorganic and, organic plant foods, naturally present or added as fertilizers, and to supply! Them, as needed, to growing plants.

The kinds and quantities of raw materials suitable for the production of plants vary in different soils and, consequently, the kinds and quantities of plants that different soils can produce vary. Some soils contain substances which interfere with the natural chemical processes involved in plant growth and thus produce abnormal or unhealthy plants or alter the resulting products so that they are not suitable raw materials for the chemical processes involved in animal growth or sustenance. Moreover, some soils are deficient in one or more of the raw materials required for the normal course of the chemical processes. Involved in the production of certain plants. Such deficiencies may prevent the production of particular plants entirely, they may result in abnormal or unhealthy plants, or. They may so alter the composition of the resulting products that they cause unhealthy conditions in animals consuming them as food.

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