BOZEMAN — Sulfur’s role as an agricultural nutrient is often overlooked, but including it in soil fertility programs can positively impact crop yield, quality and economics, according to a Montana State University Extension specialist.

Clain Jones, MSU Extension soil fertility specialist and professor in the Department of Land Resources and Environmental Sciences, said that modern phosphorus and potassium fertilizers contain less sulfur as a manufacturing byproduct than they historically did. Coupled with higher-yielding varieties and more intense cropping, leading to more sulfur being removed from fields, there is an increased chance for sulfur deficiencies. Sulfur-deficient plants are often stunted with yellow upper leaves, in contrast to yellow lower leaves, which indicates nitrogen deficiency, Jones said.

"Crops can respond to sulfur additions in a manner similar to nitrogen," Jones said.

Sulfur, like nitrogen, is a building block for proteins. If sulfur is deficient, crops may not use nitrogen fertilizer as efficiently, resulting in less than optimum grain protein and yield. By applying sulfur and increasing the efficiency of nitrogen use, farmers can also decrease nitrate leaching and soil acidification — and possibly lower the amount of nitrogen needed, Jones said.

Testing

Jones said there is no single recommended sulfur soil test as there is with nitrate. That’s partly because soil sulfur availability is often highly variable across a field. Coarse-textured soils with low organic matter are the most susceptible to sulfur deficiencies. He noted that when six to 10 soil samples from across a field are mixed for laboratory analysis — which is the standard in Montana — one or two high-sulfur samples could skew the results and suggest the field doesn’t need sulfur when most of it is deficient.

“Although more expensive in the short term, it can be highly beneficial to learn which parts of a field have low sulfur through ‘grid sampling’ instead of mixed samples,” Jones said.

Jones recommends learning what sulfur test a laboratory uses. He said numerous soil tests are available, and each can produce very different results.

“For example, the Mehlich-3 test, which is used in midwestern and eastern states, greatly overestimates sulfur availability in higher-pH Montana soils,” Jones said. Calcium phosphate extraction tests are recommended for western soils, but other extracts might do an adequate job of assessing available sulfur, too, Jones said.

In addition, Jones recommends testing sulfur fertilizer in strips within fields and conducting plant tissue sampling to learn if crops have sulfur deficiencies. Plant tissue testing should be done early in the season so that a “rescue” sulfur treatment can be applied to nitrogen-deficient areas.

Fertilization

Jones recommends considering the “4Rs” – right place, right rate, right time and right source – for fertilization.

Because sulfur is needed early in the growing season, it should be applied at seeding, either in the furrow or side-banded. Side-banding avoids the risk of fertilizer damage to the seedling when high rates are placed in-furrow, he said. Sulfate is mobile and can also be applied as a surface broadcast application if ammonium sulfate is used.

Gypsum is less acidifying than ammonium sulfate, so Jones recommends using it on acidic soils, even though it’s somewhat less soluble. Elemental sulfur, on the other hand, becomes available too slowly for in-season use and needs to be applied a year or two before it is needed. Fall applications of sulfate are not recommended because sulfur, like nitrogen, can be leached easily by fall-to-spring precipitation.

Unlike other major nutrients, there’s limited guidance for sulfur fertilization rate requirements of Montana crops. Oilseeds, including canola and yellow mustard, need far more sulfur than cereals like wheat and barley, Jones said. He noted that in high-yielding oilseed fields, 20 pounds of sulfur fertilizer per acre should prevent sulfur deficiency, regardless of soil test results.

Alfalfa is also a large user of sulfur, needing about 22 pounds of sulfur per acre for a 4-ton per acre yield. Cereals and pulse crops, like lentils, chickpeas and peas, likely need only 5-10 pounds of sulfur fertilizer per acre, with the higher amount needed on fields with high yield potential.

Research

A recently accepted research paper by Perry Miller, professor in the MSU Department of Land Resources and Environmental Sciences, to be published in the Agronomy Journal, found that only 5 pounds of added sulfur per acre increased lentil grain yield in 20% of sites in Montana and North Dakota. Due to the low cost of sulfur fertilizer, the average increased grain yield easily offset the sulfur fertilizer cost, even when accounting for sites where lentil grain yields did not increase, Jones said.

Jones is currently conducting research funded by the Montana Fertilizer Check-off with colleagues Miller, Pat Carr and Justin Vetch with the MSU Department of Research Centers to determine the sulfur needs of spring canola, pea and wheat, and to evaluate different sulfur soil tests, following previous MSU studies on the effects of sulfur on lentil crops. In addition, there are ongoing Montana sulfur studies on winter canola led by Miller and on cereal forages led by Hayes Goosey in the MSU Department of Animal and Range Sciences.

Goosey noted that adequate sulfur levels can decrease forage nitrate levels in cereal forages. 

“Forage nitrate is a concern for livestock owners because it causes early-term abortions and other reproductive issues in pregnant animals," Goosey said.  

Questions about sulfur or soil nutrients may be directed to Jones at clainj@montana.edu or 406-994-6076.