Iron and Soil Fertility

Fertilizers that are essentially sulfurfree have been used increasingly. Ammonium nitrate, anhydrous ammonia, urea, and ammonia solutions are examples among the nitrogen carriers. The use of concentrated superphosphate and other phosphorus carriers with little sulfur has been expanding. We recommend that farmers in areas of potential deficiency should have a planned program of adding sulfur instead of depending on incidental applications in ordinary fertilizer materials. Losses of sulfur from soils are due mainly to leaching, erosion, and removal by crops. Some sulfur is lost from soils of humid regions in drainage water. Measured losses from eight soil types in Illinois maintained in fallow ranged from 1.5 to 57 pounds of sulfur an acre a year; the average was 30 pounds. The larger losses occurred from the more permeable and more fertile soils. Annual losses from a Fayette silt loam with 10 percent slope in Wisconsin were approximately I pound an acre when crops were grown and 3 pounds an acre from uncropped soil. The measurements in both States were made in lysimeters that permitted runoff and probably are fairly applicable to sloping upland soils. Earlier measurements, made in lysimeters that did not permit runoff, were considerably higher. Sulfur losses from erosion have been estimated to average about 6 pounds an acre. The losses may deplete soil supplies of sulfur, except perhaps in industrial and irrigated areas where accretions may exceed losses. A balance sheet of the sulfur economy of soils and crops in the United States would appraise these additions and losses. It would be expected that soils well supplied with organic matter and occurring near industrial areas would not at present need sulfur fertilization. Requirements in such areas may change, particularly if sulfur-free fertilizers come into wide use or coal is replaced as fuel. Iron and