The role of sulfur in pastures

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cattle on pasture
(Farm and Dairy file photo)

Sulfur is an element found in two out of the 20 protein-forming amino acids in plants. It is also essential for chlorophyll production, the most important pigment in the plant kingdom.

Additionally, a higher amount of sulfur is needed in legumes for nitrogen fixation. Legumes are thought to be the most sensitive plants to sulfur deficiency due to the fact it will slow down all three essential functions in this category.

For these reasons, plants, and all life for that matter, need sulfur to survive. It is considered a secondary macro-nutrient because of its essential requirement at lower levels than the other macro-nutrients: nitrogen, phosphorous and potassium. Plant symptoms of sulfur deficiency are young leaves with light green veins and interveinal areas. Younger leaves due to protein synthesis and light green due to decreased chlorophyll content.

Goals

There is a distinction when it comes to amending soil for sulfur; are you adjusting the pH or adding S as a nutrient? Elemental sulfur (H₂SO₃) takes several months to react and will cause acidification. There are times when this is important, especially in alkaline soils found in western Ohio.

The other common option is sulfate found in many fertilizers like ammonium sulfate also known as “AMS” (NH₄)₂SO₄ and gypsum (Calcium Sulfate). Sulfate is readily available for plant uptake unlike elemental sulfur which needs to be oxidized by soil microbes and takes several months to convert to a plant usable form.

This oxidation process of elemental sulfur also drops the pH of the soil. Sulfate is the available form of the S nutrient and is found to be more prevalent in areas of high organic matter. You are more likely to see sulfur deficiencies in coarse, sandy soils due to sulfate leaching.

Historically, eastern Ohio had some of the highest sulfur content soil in the state due to the impact of coal energy production. After the impacts of acid rain were discovered and mitigated, less sulfur was found on the soils of eastern Ohio.

Testing

Many soil testing labs have established a threshold of 15 ppm (Mehlich 3), as the critical level for sulfur in the soil. Currently, we have seen a shift in the amount of sulfur in eastern Ohio as soil tests are coming back deficient (less than 15 ppm) in S. When amending the soil, elemental sulfur and sulfate are the two easiest forms to add sulfur to the ground.

Most forage analysis laboratories will test for S in tissue samples. Many people consider this a more accurate test of S deficiency due to its low soil concentration in the soil. Much like analyzing for micro-nutrients, a tissue sample analysis that can be obtained through a hay forage analysis can indicate S deficiencies. Leaf concentrations ranging from 0.2%-0.5% are considered adequate by most labs.

The work and cost required to amend S might make a forage analysis practical in this case. “Best management practice” for feeding hay to livestock is to have a laboratory forage analysis done to optimize nutritional animal needs. Remember that all life requires S, so if the dried grass is deficient, the animal could also be deficient. This is just another tool we have available for pasture fertility.

Amendments

Manure is a great way to add micronutrients to a field in a well-balanced and cost-effective manner. Manure is also rich in organic sulfur and easily converted to sulfate through soil-microbe chemistry.

If a forage analysis is done to monitor field forage quality, it can also be a tool to determine S-deficient fields that need to be amended. A basic price analysis with urea priced at ~$500 per ton and granular AMS priced at ~$425 per ton would cost $0.54 and $1.01 per unit N, respectively.

If manure is not available, it is almost twice the cost to fertilize a field with sulfur utilizing AMS. At this price, a forage analysis would make sense before such a large investment. Furthermore, AMS is not always available at your local co-op and would take even greater effort to obtain.

My concluding thoughts are to adjust the pH needs of a field first with either agricultural lime or elemental sulfur- if acidification is required. Secondly, address macro-nutrient needs of N, P and K. Finally, I would recommend examining S levels of a field by utilizing a forage analysis in addition to a soil test.

Chances are if you have addressed pH needs and macro-nutrients, a soil test has already been accomplished. If the S levels are below 15 ppm in a soil test, a forage analysis would provide a definitive answer for sulfur needs. A sulfur level of 0.2%-0.5% in a forage analysis should be adequate for plant needs.

Also, remember that manure is a great way to add sulfur if AMS is difficult to obtain and/or if manure is more cost-effective.

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