On another field at Rothamsted, England, the average yield of barley for the same sixty years was 43 bushels an acre where nitrogen, phosphorus and calcium were regularly applied, 42.6 where all five elements--including potassium and magnesium--were added, but only 14.3 on unfertilized land. On still another Rothamsted experiment field, where a four-year crop rotation of turnips, barley, clover (or beans) and wheat has been practiced since 1848, the yield of turnips in 1908 was 717 pounds an acre on unfertilized land and 35,168 pounds where the five important elements of plant food had been regularly applied once every four years--for the turnips only--since 1848. In 1909 the barley yielded 33.4 bushels an acre on the fertilized land, but only 10 bushels where no plant food was applied. The yield of clover in 1910 was 8590 pounds an acre on the land fertilized for turnips, but only 1949 on the unfertilized land. The wheat following the clover with no other fertilizer produced 24.5 bushels an acre in 1911, but 38 bushels where plant food is always applied for turnips grown three years before. These are the established facts from the longest accurate record, and thus the most trustworthy data the world affords; and when one hears promulgated the very pleasing doctrine that the rotation of crops will maintain the fertility of the soil it is time to remember that "to err is human." Fertility in Normal Soils Of the four important mineral elements, potassium is by far the most abundant in common soils. Thus, as an average of ten residual soils from ten different geological formations in the eastern part of United States, two million pounds of subsurface soil were found to contain: Potassium 37,860 pounds Magnesium 14,080 pounds Calcium 7,810 pounds Phosphorus 1,100 pounds Even the depleted, and to some extent abandoned, gently undulating upland "Leonardtown loam," which was farmed for generations and which, according to the surveys of the Federal Bureau of Soils, covers 41 per cent of St. Mary's County, Maryland, and more than 45,000 acres of Prince George's County--still contains in two million pounds of surface soil--corresponding to the plowed soil of an acre about 6-2/3 inches deep: Potassium 18,500 pounds Magnesium 3,480 pounds Calcium 1,000 pounds Phosphorus 160 pounds The brown silt loam prairie soil of the early Wisconsin glaciation is the most common type of the greatest soil area in the Illinois Corn Belt. Two million pounds of this surface soil contain as an average: Potassium 36,250 pounds Magnesium 8,790 pounds Calcium 11,450 pounds Phosphorus 1,190 pounds The older gray silt loam prairie, the most extensive soil of Southern Illinois, contains in two million pounds of soil: Potassium 24,940 pounds Magnesium 4,690 pounds Calcium 3,420 pounds Phosphorus 840 pounds