Fertility in Illinois soils.—The upper 63 inches on an acre of fertile soil in good physical condition contains a total of not less than 8,000 pounds of nitrogen, 2,000 pounds of phos phorus, and 30,000 pounds of potassium. With these numbers in mind, the farmer can determine the elements in which his land is deficient if he has at hand the report of the Soil Survey giving total amounts of nitrogen, phosphorus, and potassium per acre in lands belonging to the same soil type as the farm under consideration. The table on page 156 from Bulletin No. 123 of the Agricultural Experiment Station gives the fertility in the various soil areas and soil types most widely represented in the state.
The first column of the table indicates the soil type by number. The figures representing hundreds correspond to the soil areas of the state as numbered on the soil map, and the name of the soil area is given in the second column. The two right-hand figures of the first column stand for a soil type, which is named in the third column. Thus 30 stands for "gray silt loam on tight clay," while 330 tells that this gray silt loam on tight clay is in the soil area indicated on the map by the number 3, the Lower Illinoisan glaciation. The number 26 stands for "brown silt loam"; but 426 tells that this brown silt loam is in the Middle Illinoisan glaciation; 526 locates it in Upper Illinoisan glaciation; and 1126 in the Early Wisconsin.
Columns 4, 5, and 6 show the total nitrogen, phosphorus, and potassium present in one acre of surface soil to a depth of 61 inches. By comparing these amounts with the minimum amounts given above for an acre of fertile soil, it is easy to see whether the soil type under consideration is deficient in one or more of these limited elements of plant food. Column 7 shows the amount of limestone required to correct the acidity of the soil.
Thus by turning to the map we see that a farm in Sangamon County is in soil area No. 4, the Middle Illinoisan glaciation. By examining the table we learn that if this Sangamon County farm is made up of undulating prairie lands it belongs to soil type No. 426, "brown silt loam," and that the upper 61 inches of an acre of this soil contains 4,370 pounds of nitrogen, 1,170 pounds of phosphorus, and 32,240 pounds of potassium. By comparison with the requirements of a fertile soil we find that this land is deficient to the amount of 3,630 pounds of nitrogen and S30 pounds of phosphorus per acre, while there is an excess of potassium. If the farm lies in the flat prairie lands
of Sangamon County, it belongs to soil type No. 420, "black clay loam," and the deficiencies for nitrogen and phosphorus are somewhat smaller than in the "brown silt loam." If the farm lies in the rolling or hilly timber uplands of Sangamon County, it belongs to soil type No. 435, "yellow silt loam." The deficiencies for nitrogen and phosphorus are very great, and a liberal application of ground limestone is required.
The county soil report for Sangamon County shows the soil types of our selected farm accurately mapped to ten-acre areas, and the report contains definite information as to the best methods of increasing the fertility in the various soil types.
The table shows that the nitrogen content of the surface soils per acre varies from 1,440 pounds in the sand soil to 34,880 pounds in deep peat; the phosphorus, from 810 pounds in yellow-gray silt loam to 2,030 pounds in black clay loam; potassium, from 2,930 pounds in deep peat to 47,600 pounds in yellow-gray silt loam. Only two of the soil types given in the table have an excess of nitrogen over the minimum amount indicated for a fertile soil; only one has an excess of phos phorus; while all but two have an excess of potassium.
Agricultural experiment fields.—Agricultural experiments are of real worth only as valuable and practical results are made available to the farmers and used by them. Scientific soil treatment is brought directly to the farmers of Illinois in their home localities by means of agricultural experiment fields. These fields are established in all parts of the state on various kinds of soil typical of the regions in which the fields are located. The experiment fields are operated under the ordi nary crop conditions of the locality, and the farmers may learn the effect of various methods of soil treatment by ob serving the field in their locality during the season and on special occasions when a repre sentative of the Agri cultural Experiment Station visits the field to discuss the results with the farmers of the neighborhood. Information concerning these experiment fields has been obtained from various publi cations of the experiment stations.