Sulfur (S) is considered a secondary macronutrient, following the primary macronutrients nitrogen, phosphorus, and potassium, but is needed by plants at levels comparable to phosphorous. Plants require S for synthesis of cystine, cysteine and methionine, which are amino acids that form an integral part of proteins. Additionally, S is required in the formation of vitamins, enzymes, chlorophyll and plays a vital role in basic plant functions like photosynthesis and nitrogen fixation.

Sulfur has received more attention due to increasing areas of S deficiency since the enactment of the US-Clean Air Act in 1970 and its subsequent implementation to remove S from coal and oil-based industries. Across the USA, average atmospheric sulfur dioxide (SO2) values in the air has been reduced by 87% from 1980 to 2016 (U.S. Environmental Protection Agency, 2017). Soil S levels have decreased steadily as S removal, crop yields have increased, and deposition of SO4-S via rainfall, fertilizer, and pesticides has decreased. Unlike N, P and K in the central USA, researchers have not studied S extensively mainly because S deficiency symptoms were seldom seen outside of very deep, low organic matter, sandy-textured soils. However, in recent years the incidence and severity of S deficiency has been observed in the central states of the USA, including North Dakota and Minnesota.

According to current recommendation in North Dakota, at least 10 lb S ac-1 should be applied if rainfall or snow-melt is above normal in low organic matter soils in fall, winter, or early spring. The goal of this study was to revisit corn S fertilizer recommendation for the Red River Valley region.

This experiment was conducted at five sites- Ada, Amenia, Casselton, Downer, and Walcott in 2017 growing season. Five S treatments (check, 10, 20, 30 and 40 lb S ac-1) were applied using ammonium sulfate in randomized complete block design with four replications. The objectives were 1) to evaluate corn response to five S application rates, 2) to determine relationship between corn yield and leaf S analysis.

Corn yield increased with S fertilizer application at one site (Downer) in 2017 growing season. Yield increase was observed with 40 lb S ac-1 application rate. None of the other sites showed a yield increase with fertilizer S application. In addition, leaf S correlated poorly with the corn yield. The lack of response at the remaining sites showed that corn S needs were fulfilled by other S sources. The rainfall in the initial growth months (May-June) of corn was very low compared to normal rainfall, which may have resulted in less S leaching from the soil. Since the North Dakota sub-soils are rich in sulfate salts, this dry season could have led to S accumulation in surface layer with upward movement of water and its solutes. The S from sub-soil combined with mineralized S from organic matter might have been enough for optimum growth of crop. More sites may have responded to S if the initial growing season had higher rainfall. These results indicate that corn yield response to S varies with soil and weather conditions, and corn leaf S concentration is a poor predictor of grain yield response.