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Sulfur: The 4th Macronutrient

Nitrogen, phosphorus and potassium (NPK) have historically received the lion’s share of attention as the “Big 3” macronutrients. Yet as North American farmers seek to optimize crop production and maximize yield potential, sulfur (S) is receiving its due as the fourth essential macronutrient.

Elemental sulfur (primarily sold in a sulfur-bentonite pastille) provides an economical, high-analysis, season-long source of S that is highly compatible in a variety of fertilizer blends. It can be applied as part of a long-term soil management plan to address S deficiency, or used to lower soil pH as part of an amendment program.

“Historically crops got their sulphur from the environment via SO2 gas from industrial processes entering the sulphur cycle in large quantities.”* Between 1990 and 2019, North American emissions have been reduced by 90%!

* The Sulphur Institute

Addressing Sulfur Deficiency

The need for nutritional sulfur has increased dramatically over the past four decades. Strict regulations limiting industrial sulfur emissions have helped address a major source of environmental pollution. Yet removing a once abundant supply of sulfur has led to nutrient deficiencies in crops throughout North America: increasing the need for sulfur fertilizers.

Coarse-textured, soils that are well-drained and low in organic matter are most prone to sulfur deficiency. Soil sampling and tissue testing can both help identify deficiencies.

Different crops show different signs of deficiency. Since sulfur is essential to chlorophyll production, one common sign is a yellowing of the leaves. Soil and tissue tests can provide hard evidence to guide your nutrient management strategy.

The Role Of Sulfur As A Crop Nutrient

Sulfur is essential to plant health and development at multiple key growth stages.

Photosynthesis depends on sulfur, which is required to produce chlorophyll. Plants also require sulfur to synthesize starch, sugars, oils, fats and vitamins and rely on sulfur to metabolize nitrogen. Sulfur plays a central role in synthesizing oils in oilseed crops.

Oilseeds, legumes, forages and certain vegetable crops have a high sulfur demand.

How Elemental Sulfur Becomes Plant-Available In The Soil.

Plants need sulfur, but it is not plant-available in its elemental form. In order for plants to absorb sulfur, it must be first converted to a soluble sulfate (SO4). Fortunately, soilborne microbes are ready and able to perform this role. The microbes oxidize the S. Once the conversion is complete, plants are able to turn the sulfate to organic compounds they require.

How Bentonite Clay Optimizes Sulfur Conversion.

Combining bentonite clay with elemental sulfur speeds up the conversion process. The bentonite clay in Keg90S™ and Keg85S™ absorbs water and breaks down the pastilles and sulfur crystals into small particles. This provides a large surface area that encourages colonization by the bacteria that oxidize elemental sulfur, to facilitate a faster conversion rate to SO4.

Premium bentonite clays that have a higher capacity to swell when moisture is present to provide faster, more consistent breakdown of the pastille and sulfur particles – and more rapid oxidization.

And the higher the clay content, the more efficient the conversion to SO4. Keg River was the first to introduce an 85-analysis bentonite sulfur, with 50% more clay content than the standard 90-analysis product. Others have since followed our lead.

Application Timing Of Bentonite Sulfur.

Since elemental sulfur requires sufficient time to convert to plant-available sulfates in the field, it is often applied in the spring, in conjunction with ammonium sulfate (AMS) which is immediately plant available, and can address the early plant demand for sulfur. The elemental sulfur provides a steady supply of sulfur throughout the remainder of the growing season (at a more economical price point).

By applying bentonite sulfur in the fall, there may be little to no need for AMS, as the conversion process has begun and SO4 is available at the earliest growth stages.

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Sulfur Benefits By Crop.

While all plants require sulfur, some crops have a higher demand than others. Let’s take a look!


Few crops have a greater requirement for sulfur than oilseeds, such as canola, soybean, linseed, sunflower and flax. Sulfur is vital to the synthesis of oils and has an impact on the yield, quality and economics of the crop. Canola has one the highest sulfur uptake requirements: at roughly 0.6 lb./bushel. Sulfur demand is highest during flowering and seed set. As a season-long source of sulfur, bentonite sulfur fertilizers provide a dependable supply of this key nutrient at this critical juncture.


In legume crops, sulfur plays an important role in facilitating nitrogen fixation and helps legumes form nodules on root hairs. Soybeans, chickpeas, dry beans, lentils and peanuts all benefit from a fertilizer program that delivers the necessary sulfur requirements. Soybean and peanuts can be used for oil production, and benefit from sulfur.

Corn & Alfalfa

Alfalfa and corn are two forage crops that require ample amounts of sulfur to maintain healthy growth and achieve optimum yields.

A bushel of corn will remove 0.17 lb. of sulfur (0.08 lb. in the grain and 0.09 lb. in the stalk. Research has shown that corn plants uptake roughly 10 percent of sulfur by the V6 growth stage, 50 percent between V6 and silking, and 40 percent from silking until crop maturity. Signs of sulfur deficiency included slow growth, and yellow striping on the leaves, along with a light green whorl.

A 4 ton/acre alfalfa crop will remove approximately 20 lbs/ac of S. According to OMFRA, a single application of 50 lbs/ac of elemental sulfur will typically last the life of a productive 3 year alfalfa stand. Sulfur-deficient alfalfa will appear light/yellowish green and spindly with stunted growth, compared to a healthy crop.

Root Crops

Sulfur fertilizers are widely used in the production of root crops, including potatoes. Sulfur is considered an important nutrient for those seeking to optimize potato production. To achieve 450 cwt yield, potatoes require 20 lbs sulfur per acre. Since potatoes tend to grow best in low pH soils, sulfur may be used to amend calcareous and saline/alkali soils. In potatoes, sulfur deficiency is indicated by a yellowing of leaves. Leaflets may show a slight upward roll.

Grain sorghum (milo)

Sulfur is known to improve nitrogen use efficiency in grain sorghum, particularly in high yield production systems. It is recommended that growers try to maintain a nitrogen to sulfur ratio of 15:1.


Vegetables such as cabbage, broccoli, cauliflower, kale and Brussels sprouts are rich sources of dietary sulfur (which explains the slightly bitter taste). Sulfur fertilizer is key to the success of these crops.

For more information on sulfur deficiency by crop, click here.

Download the Keg River Sulfur Uptake Chart

A free download to help you calculate sulfur needs by crop.