Sulfur for Mississippi Crops

Sulfur in Mississippi Crop Production

Sulfur is required in fairly large quantities by most crops. It is an essential building block in chlorophyll development and protein synthesis. Sulfur is also required by the rhizobia bacteria in legumes for nitrogen fixation. In general, crops remove about as much sulfur as phosphorus. Grasses remove sulfur more efficiently than legumes, and clovers often disappear from pasture mixtures when sulfur is low. Sulfur nutrition of wheat has been an issue on some soils.

The sulfate ion, SO4, is the form primarily absorbed by plants. Sulfate is soluble and is easily lost from soils by leaching. As sulfate is leached down into soil, it accumulates in heavier (higher clay content) subsoils. For this reason, reliable soil tests for sulfur in topsoils in humid regions such as Mississippi have been elusive for predicting sulfur availability for crop use during a long growing season. However, some mid-south Extension Services report critical levels for their state.

We know from experience that many coarse textured, sandy soils, and low organic matter silty soils throughout Mississippi may be sulfur deficient for crop production. Many acid soils contain metallic sulfides that release sulfur as weathering occurs. Organic matter is the source of organic sulfur compounds and is the main source of soil sulfur in most Mississippi soils.

In recent years, the National Atmospheric Deposition Program data shows about 10 pounds of sulfate currently deposited per acre in rainfall annually in Mississippi. This has decreased from 13 to 15 pounds per year in the not-so-recent past due to improved industrial scrubber technology, automobile catalytic converters, and changes in fuel composition.

Recent MAFES research found a positive yield response of soybeans to ammonium sulfate only on the sandier location of several trials. Auburn research found row crop response to sulfur applications in only ‘worst-case’ scenarios of low organic matter, coarse textured soils, in dry years. Recent work in Louisiana found responses by wheat to applied sulfur (as ammonium sulfate) on alluvial, coarse-textured, very fine sandy loam soils, but not on finer textured upland soils.

Sulfur deficiency symptoms show on young leaves first, and the leaves appear pale green to yellow in color. The plants are spindly and small with retarded growth and delayed fruiting. Some early season deficiencies do not carry forward to reduced yields as soil mineralization rates of organic sulfur to sulfate form increase as temperatures increase. For a rapid correction of a deficiency, use one of the readily available sulfate sources.

Sulfur provided by the MSU Extension Service Soil Testing Laboratory for row crops is based on the organic matter content of the sample. Historically, the general sulfur recommendation for major crops in Mississippi is eight pounds per acre annually where soil sulfur is suspect (the Alabama recommendation is 10 pounds per acre). Another recommendation system used has been applying 10 pounds of sulfur for each 100 pounds of actual nitrogen applied.

Therefore, site specific characteristics need to be considered for sulfur amendment: organic matter content and texture of the soil, the particular crop (whether a grass), spring seeded versus fall seeded, and water management.

Several sources of fertilizer sulfur are available commercially. Readily available (in the plant nutrition sense of available) sources include ammonium sulfate, potassium sulfate, gypsum, and zinc sulfate. There are several other sulfate sources as well as less available sources of sulfur in the elemental or sulfide form. Because plants utilize sulfur in the sulfate form, elemental sulfur should be applied well before growing plants will need it so it will be oxidized to sulfate which can take up to four months.

Elemental sulfur is a good acidifying agent. An application of 500 pounds of sulfur per acre on sandy loam soil reduces the pH from 7.5 to 6.5. It takes about three pounds of lime to neutralize the acidity formed by one pound of sulfur.