Note 1 — Summary of Bulletin No. 94.

It should be remembered that there are ten essential elements of plant food each of which is of equal importance to the plant, for if the plant is deprived of any one of the ten essential elements it is impossible for it to develop and mature. Carbon has no market value as plant food because the plant obtains carbon in the form of carbon dioxid, a gas which is present everywhere in the atmosphere and which the plant inhales through its leaves. Both hydrogen and oxygen are without market value because they are the elements which compose water, a liquid compound which plants absorb through their roots. Calcium, magnesium, iron, and sulfur have no market value as elements of plant food because they are present in practically all soils in abundance as compared with the amounts required in plant growth. The three elements nitrogen, phosphorus, and potassium, do have market values, because they are required by plants in very considerable quantities, and they are present in most soils in rather limited amounts, and when the available supply of any one of these elements becomes too much reduced in a soil the crop yield also becomes reduced. For further information regarding the use of these elements of plant food on Illinois soils, see Circular No. 68, “Methods of Maintaining the Productive Capacity of Illinois Soils.”