But the other five elements require careful consideration if lands are to be kept fertile. These are potassium, magnesium, calcium, phosphorus and nitrogen; and every landowner ought to be as well acquainted with these five elements as he is with his five nearest neighbors.
Instead of making this acquaintance and gaining a knowledge of important facts and principles, the average farmer in the older states, with failing fertility, has made the acquaintance of the fertilizer agent; and instead of purchasing what he needs for the permanent improvement of his soil, he buys what the agent wants to sell, with the common result that the seller is enriched while the soil is merely stimulated to greater poverty.
Potassium.—A careful study of the facts shows that potassium is one of the abundant elements in nature; that the average crust of the earth contains 212 per cent, of this element, and that normal soils bear some relation in composition to the average of the earth's crust.
If normal soil had the same percentage, then the plowed soil of an acre 623 inches deep (corresponding to 2 million pounds of soil) would contain 50,000 pounds of potassium. In Illinois, the normal soils actually do contain from 25,000 to 45,000 pounds per acre of this plantfood element in the first 623 inches, while less than 4 pounds of potassium would be added in an application of 200 pounds of the most common commercial fertilizer. The Illinois system of permanent fertility does not provide for the purchase of potassium for normal soils, but it does provide for the liberation of an abundance of that element from the practically inexhaustible supply in the soil. This liberation is accomplished by the action of decaying organic matter plowed under in the form of farm manure or crop residues, including clover or other legumes.
Only where the soil is positively deficient in potassium susceptible of liberation, as is the case with some sand soils and with most peaty swamp lands, need potassium be purchased in permanent systems of either grain farming or live-stock farming; but in market gardening or in raising timothy hay for the market commercial potassium may be required; and, on some worn soils especially deficient in decaying matter, temporary use of kainit is often advisable.
Magnesium and Calcium.—As a general average, normal soils contain more than four times as much potassium as magnesium, while the loss by leaching and cropping in rational systems of grain or live-stock farming may be actually greater for magnesium than for potassium, so that magnesium is more likely to become deficient in soils than is potassium.
The calcium supply in normal soils is also only one fourth that of potassium, while the average loss by cropping and leaching is four times as great, so that 16 to 1 expresses the relative importance of calcium and potassium in the problem of permanent fertility on normal soils.
All limestones contain calcium; and the common dolomitic limestone
