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Sulfur (S) deficiencies are more common due to less atmospheric deposits, declining organic matter levels and bigger crop yields. It’s time to add sulfur to the list of important nutrients so the “Big 3” now becomes the “Big 4” – nitrogen (N), phosphorus (P), potassium (K) and S. According to the International Plant Nutrition Institute (IPNI), a 50 bu/A soybean crop needs 23 lbs. of S while a 75-bushel crop will need about 30 to 35 lbs.

TABLE 1 - Nutrient requirements for 50-bushel soybeans:

http://www.ipni.net/ppiweb/ppinews.nsf/0/2aa6a1b33813f7cb8525691c005bd1ae/$FILE/98105-NRU-Soybeans.pdf

Soybeans take up the sulfate (SO4) form of S throughout the entire growing season, with relatively constant uptake beginning with pod set (R3). Sulfur is immobile within the plant, which means the plant is unable to compensate for poor S availability that may occur late in the season by cannibalizing sulfur from older leaves to new leaves.

The most common forms of S available are elemental sulfur, ammonium sulfate (AMS), calcium sulfate (gypsum) and liquid thiosulfates. Sulfates are mineralized from organic matter at a rate of about 1 lb. of S for 7 lbs. of N. Elemental sulfur is broken down into sulfate over time by microbes and the rate of breakdown is determined by soil conditions, microbial population, temperature, moisture and incorporation.

An ILSoyAdvisor follower recently posed the question, “What is the release rate of the 10 lbs. of (elemental) sulfur in a Hoytville clay environment with a pH of 6.2?” The reader asked the question after listening to an ILSoyAdvisor webinar presented by Wesley Haun, agronomist with H.J. Baker & Bro.

Haun said, “Sulfur reaction in soil resembles nitrogen and is difficult to predict, as it is influenced by microorganisms. Such factors as soil temperature, soil moisture, population of specific microorganisms, and soil aeration influence the process. The activity of soil microorganisms increases with soil temperature and if all other factors are favorable, sulphate release is faster.”

Haun explained that the bentonite Tiger-Sul includes in their products enhance this process by reducing the particle size of the sulfur pastille (granule) into powder-size particles, which provide greater surface area for the microorganisms to feed on the sulfur and convert it to sulfate. “Our lab trials under controlled environmental conditions revealed significant sulfate release within 1 week, increasing with time when (soil) conditions are ideal,” said Haun.

Soil microbes generally like a near neutral pH to flourish, so a value of 6.2 is near neutral and microbial activity won’t be suppressed. However, S can impact soil pH. “Elemental S can be used as a soil amendment to reduce soil pH,” said Haun. “The amount of S required to lower soil pH is a function of the soil buffering capacity and texture. A medium textured soil will require approximately 600 lbs./A of elemental S to reduce the soil pH by one point, from 7.5 to 6.5, when incorporated 6-inches deep and when no free calcium carbonate is present. When elemental S is applied at plant nutrient rates of 30 lbs./A, I anticipate very minimal influence on actual soil pH. If my math is correct 600 lbs. will reduce pH 1 point while 30 lbs. would potentially reduce soil pH 0.05 of a point.”

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.