Two others, magnesium and calcium, are so abundant in Illinois limestones that their supply at moderate cost is assured for all time. Special application of ground rock for magnesium is rarely necessary. Large quantities of ground limestone are used annually on Illinois soils, and greatly increased crop yields arc thereby obtained. It is especially necessary where the soil is "sour" or acid. The limestone is required much more for the purpose of correcting the acidity of the soil than as a plant food. Since it does play so large a part in crop yields, however, it is to be considered as one of the important elements of plant requirements which must have the intelligent attention of the farmer.
The three remaining elements, nitrogen, phosphorus, and potassium, are required in considerable quantities by all crops, while in most soils the supply of one or more of them is limited. If the supply of one of these elements is too limited, it must, as a consequence, limit the yield of the crop, even though all other factors essential to crop production arc well provided.
The actual condition is illustrated by the following examples : The sand-ridge soil of Tazewell County produced, without soil treatment, crops valued at $12.90 per acre per year as the average for six years; with nitrogen added the yields were valued at $19.51 per acre per year; additions of potassium with the nitrogen increased the yield to $23.53 per acre per year. The prairie soil of McLean County yielded, without soil treatment, crops valued at $15.83 per acre per year; with phosphorus added the yields were valued at $20.73 per acre per year; additions of nitrogen and potassium with the phos phorus increased the yield to $22.77 per acre per year. The peaty swamp lands of Kankakee County yielded, without soil treatment, crops valued at 70 cents per acre per year; with potassium added the yields were valued at $13.89 per acre per year; additions of nitrogen and phosphorus with the pot assium gave yields valued at $15.44 per acre per year. The yellow silt loam hill land of the unglaciated area in Johnson County produced, without soil treatment, crops valued at $4.19 per acre per year; with legumes added, $5.12 per acre per year; with legumes and lime, $10.41 per acre per year; and with legumes, lime, and phosphorus, $12.62 per acre per year. These values are all based on prices which were extremely low as compared with the war-time prices of 191S.
These illustrations, based on actual field experiments, show conclusively that the sand-ridge soil is especially deficient in nitrogen, the prairie soil in phosphorus, the peaty soil in potassium, and the unglaciated soil in calcium as well as other elements.
The problem of a per manent and profitable agriculture on Illinois farms may be expressed in briefest form by the formula LNPK, in which L stands for limestone from which calcium is obtained; N, for nitro gen; P, for phosphorus; and K, for potassium (Lat. ka/ium). These letters, singly and in various combinations, are used on the markers in the numer ous agricultural experiment fields of the state to indicate the method of soil treatment applied to the experimental plots. These four elements, constituting less than 4 per cent of the weight of the mature crop, are the factors of soil fertility that require the intelligent consideration of the farmer.
An inexhaustible supply of calcium is found in Illinois lime stones, and it may be readily procured. Leguminous plants such as clover, sweet clover, alfalfa, cowpeas, and soy beans, on whose roots nitrogen-gathering bacteria thrive, may be grown in crop rotations and plowed under. A perpetual supply of nitrogen is thus assured, if scientific methods of agriculture are adopted. Phosphorus is obtained from bone meal and rock phosphate. It must be purchased and applied to the land. The known phosphate supplies of the world are limited. The most important producing mines are in Tennessee, South Carolina, and Florida. The largest reserves yet discovered are in Idaho, Montana, and Utah. It is possible that the supply of rock phosphate may be the limiting factor in the development of a permanent and profitable agriculture through out the nations of the world. Most Illinois soils arc exceedingly rich in potassium and this element need not be given special attention in all parts of the state. In the peaty swamp lands, however, soil improvement is almost wholly dependent on the application of the potassium salts in a concentrated form. The largest potassium beds are found in Germany, and most of the world has been dependent upon these deposits for their potash supply. It is now known that our supply can be largely won from smelter fumes and the dust of cement plants.