Fertilizer Sulfur Effects on Corn Yield and Plant Sulfur Concentration

Concern for S deficiencies of corn (Zea mays L.) is common among producers and fertilizer dealers in the Midwest. This concern results from emphasis on high yields with subsequent large S needs, accompanied by a decreasing S supply from incidental sources. A survey was made of corn response to fertilizer S in Missouri. Sites were selected on the basis of high yield potential and on soils that the farmers and fertilizer dealers suspected needed S, but that had not previously been considered S deficient. During 1991 and 1992, fertilizer S rate studies were conducted at 53 sites throughout Missouri. Sulfur rates of 0, 15, 30, and 60 lb S/acre were used. Ammonium sulfate and ammonium thiosulfate were the S sources that were applied at planting or within 4 wk following planting. Grain yields exceeded 175 bu/acre at 29 of 47 site-years. Grain yield increases from fertilizer S were observed at three of 47 harvested sites, and yield decreases at five sites. However, leaf S concentrations were above previously established critical levels. Leaf S concentrations were increased by fertilizer S at 36% of the sites at the six leaf stage and at 38% of the sites at anthesis. Grain S concentrations were increased by fertilizer S at 15% of the sites. Leaf and grain S concentrations and N:S tissue ratios correlated very poorly with relative yield. Fertilizer S recommendations (none recommended for all but one site) based on soil sulfate measured at planting were incorrect for only three sites. Consequently, this research indicates no change is needed from present S fertilizer recommendations for corn groan in Missouri. Research QuestionCorn grown in Missouri on medium to fine textured soils has not historically required S fertilizer. In addition to S supplied by the soil, S has been incidentally supplied with field applied-manures, low analysis S containing fertilizers, pesticides, and from deposition in rainfall and dust. Since the S supply from these sources has been reduced from previous years, and increased grain yields have greater crop S requirements, this study sought to survey corn response to fertilizer S on soils that were not previously considered S deficient for corn. Literature SurveyMost documented corn grain yield responses to fertilizer S in the Midwest have been on coarse textured, low organic matter soils. Greenhouse evaluations have shown many midwestern soils supply inadequate S to a growing crop. An Illinois study conducted in the late 1970s measured grain yield increases from fertilizer S at five of 82 sites. In the past, S deposited in rain probably has been a significant portion of the S used by a crop. However, the amount of S deposited in rainfall in mid-Missouri has decreased by 50% in the last 10 yr, and present yield levels require more S. In recent years, first time responses to fertilizer have been observed on fine textured soils. Study DescriptionFifty-three S rate experiments were established on 25 different soil types across Missouri during 1991 and 1992. Sulfur fertilizer was applied at or near planting at rates of 0, 15, 30, and 60 lb S/acre. The S sources were ammonium sulfate (21-0-0-24 N-P-K-S) or ammonium thiosulfate (32-0-0-5). Applied QuestionDoes high yielding corn in Missouri require fertilizer S for maximum grain yield and quality? These data do not support a need for fertilizer S. Several geographic areas of Missouri, soil types, and management systems were represented by the sites. Yet none of these were identified as specific factors that relate to yield loss in the absence of fertilizer S. Grain quality (crude protein) was unaffected by fertilizer S (data not presented). When the yields of all sites were averaged, yields were similar for the zero S treatment and the S fertilized plots (Table 1). Can S fertilizer effects be predicted for corn grown on Missouri soils? Soil samples were taken at 0- to 6- and 6- to 12-in. depths at planting. Soil sulfate values varied from 0.8 to 11.7 ppm in the surface 6 in. and were often higher in the subsoil. Using present recommendation criteria, soil sulfate values accurately predicted the lack of response to fertilizer S at all but three sites. Fertilizer S increased leaf S concentrations at vegetative and silking growth stages at many sites. Leaf S concentrations were above previously established critical levels associated with S deficiencies. But under these circumstances, leaf S concentrations at these stages were not good indicators of grain yield response to fertilizer S. Results of this study suggest that corn grown on fine textured soils in Missouri obtains adequate S without S fertilization. RecommendationMissouri farmers on medium to fine textured soils do not need S in their corn fertilizer program. The frequency of crop response to fertilizer S is so low that even the cost of “insurance” S applications must be weighed against the alternative use of capital associated with such applications. Table 1Grain yields averaged across sites for 1991 and 1992. S rate, lb/acreYear0153060-------------------- bu/acre --------------------199115916016015819921841841811841991–1992174174172174