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The use of sulfur fertilizers for crop production has been thoroughly researched in Minnesota for the past 40 years. Nevertheless, studies continue for the purpose of fine-tuning recommendations for the use of this essential nutrient. Therefore, it's time for an update on the importance of sulfur for crop production.

Sulfur in Starter Fertilizers for Corn:

For the past three years, studies have been conducted where corn was grown in a ridge-till planting system for the purpose of evaluating the effect of sulfur source, placement, and rate of application on corn yield. A liquid (12-0-0-26) and dry source (21-0-0-24) of sulfur were compared. There sources were placed either in contact with the seed (pop-up) or in a band to the side of and below the seed at planting (traditional starter). The fertilizers were applied to supply 6, 12, or 18 lb. sulfur per acre.

The impact of sulfur fertilizer on corn yield was dictated by soil texture. When the soil texture was a loamy fine sand, sandy loam, or loam, the application of banded sulfur produced a substantial increase in yield. There was no response when the soil had a silty clay loam texture. Corn grown with a conservation tillage system on a soil with a silt loam texture responded positively to the application of sulfur. It is also important to point out that the soil at this site had an organic matter content of less than 2%.

The yield increase measured with the sandy soils was typical of responses measured in previous studies. The optimum rate varied with relative yield level. A rate of 6 lb. sulfur per acre was adequate when yields were in the range of 100 bu. per acre. This optimum rate changed to 12 lb. sulfur per acre as the yields increased into the range of 160 to 180 bushels per acre.

The measured response on the silt loam soil is attributed to two factors. There is very little soil disturbance in the ridge-till planting system. Therefore, smaller amounts of sulfur are released from the breakdown of the soil organic matter. In addition, the organic matter content was less than 2.0%. It is doubtful if a positive response to sulfur would have been measured if corn had been grown on the silt loam soil and a conventional tillage system had been used.

The use of 21-0-0-24 had no negative effect on yield when applied in contact with the seed. Use of the 12-0-0-26 at a rate to supply 18 lb. sulfur per acre in contact with the seed caused a reduction in emergence with a subsequent reduction in yield when corn was grown on the sandy soil. There is a risk if 12-0-0-26 is applied in contact with the seed and the soil texture is a loamy fine sand or sandy loam.

Gypsum Use and Soil pH:

Some have suggested that the application of gypsum will reduce soil pH. This is not the case for the northern and western Corn Belt. Gypsum is classified as a neutral salt and will have no effect on soil pH when applied at reasonable rates.

In the states of the southeastern United States, use of gypsum causes an increase in soil pH because of chemical reactions with soluble aluminum. In contrast to the situation in these states, soluble aluminum levels in Minnesota soils are low and use of gypsum should not have any effect on soil pH.

In the western states, application of gypsum at very high rates is used to reclaim saline (salt-affected) soils. Very high rates are required. There must be good internal drainage if this practice is to be successful. There is very poor, if any, internal drainage for fields where high salt levels are found in Minnesota

Therefore, do not expect gypsum use to have a major effect on the pH of Minnesota soils.