After ascertaining the total stock of plant food in the plowed soil, the next important question is not how much is "available," but rather how much can be made available during the crop season, year after year. In other words we must make plant food available by practical methods of liberation, by converting it from insoluble compounds into soluble and usable forms; for plant food must be in solution before the plant can take it from the soil. For the present, space is taken only to emphasize the value of decaying organic manures in the important matter of making plant food available; and attention is also called to the fact that the decomposition of the organic matter of the soil—including both fresh materials and old humus—is hastened by tillage and by underdrainage, which permit the oxygen of the air to enter the soil more freely, oxygen being a most active agent in nitrification and other decomposition processes of organic matter, as well as in the more common combustion of wood, coal, and so forth.
The Renewal of Fertility
In rational systems of general farming the supply of any element which is normally very abundant may be renewed from the subsoil by even the very slight erosion which occurs on all ordinary lands in humid sections. This statement applies to iron and potassium, and often to magnesium.
If two million pounds of normal surface soil contain 30,000 pounds of potassium, one inch an acre would contain 4500 pounds of that element; and if a third of this—1500 pounds—were removed by cropping and leaching before its removal by surface washing, then two-thirds of a century could be allowed for the erosion of one inch of soil, with crop yields of 50 bushels of wheat, 100 bushels of corn and oats, and 4 bushels of clover seed to the acre, provided the stalks, straw and clover hay were returned to the land, either directly or in farm manure. This amount of surface washing is likely to occur on land sufficiently undulating for good surface drainage, provided the land is plowed and cultivated as frequently as would be required for a four-year rotation as suggested above. Where hay, straw, potatoes, root crops or common market garden crops are sold, very much larger amounts of potassium leave the farm than in grain farming or live-stock farming, and in such cases potassium must ultimately be purchased and returned to the soil, either in commercial form or in animal manures from the cities.
Thirty Bushels for Potassium
There are some soils, however, which are not normal—soils whose composition bears no sort of relation to the average of the earth's crust; such, for example, as peaty swamp soil or bog lands, which consist largely of partly decayed moss and swamp grasses. These soils are exceedingly poor in potassium, and they are markedly and very profitably improved by potassium fertilizers, such as potassium sulphate and potassium chloride—commonly but erroneously called "muriate" of potash.
Thus, as an average of triplicate tests each year, the addition of potassium to such land on the University of Illinois experiment field near Manito, Mason county, increased the yield by 20.7 bushels more corn to the acre in 1902, by 23.5 in 1903, by 29 in 1904 and by 36.8 in 1905; and the proceedings of the midsummer session of the Illinois State Farmers' Institute for 1911 report that the use of $22,500 in potassium salts on the peaty swamp lands in the neighborhood of Tampico, Whiteside county, increased the value of the corn crop in 1910 by $210,000, the average increase for potassium being about 30 bushels of corn to the acre.
Some sand soils, particularly residual sands, which often consist largely of quartz-silicon dioxid—are very deficient in potassium; consequently the experiments or demonstrations conducted by the potash syndicate at Southern Pines, North Carolina, show very marked increases from the use of potassium salts on such soil, although the result ought not to be used to encourage the use of such fertilizers on normal soils, which are exceedingly rich in potassium.
Even in soils abundantly supplied with potassium temporary use may well be made of soluble potassium salts when no adequate supply of decaying organic matter can be provided. For this purpose, kainit—which contains potassium and also magnesium and sodium in chlorides and sulfates—is preferred to the more concentrated and more expensive potassium salts. About 600 pounds an acre every four years is a good application. The kainit will not only furnish soluble potassium and magnesium but will also help to dissolve and thus make available other mineral plant food naturally present or supplied, such as natural phosphates. When the supply of organic matter produced in crops and returned either in farm manure or in crop residues becomes sufficiently abundant, then the addition of kainit may be discontinued on normal soil.
Thus, as an average of 112 separate tests covering four different years, on the Southern Illinois experiment field on worn, thin land, at Fairfield, the use of 600 pounds an acre of kainit once in four years increased the yield of corn by 10.7 bushels where no organic manure was used, and by only 1.7 bushels when applied with eight tons of farm manure.