Some Montana soils may not have enough sulfur to support maximum crop growth, and the only way to find out is through tissue sampling, says Clain Jones, Montana State University (MSU) Extension soil fertility specialist.
Until recently, sulfur didn’t often limit crop growth in most soils, but modern N, P and K fertilizers contain less sulfur as a manufacturing byproduct. Also, higher-yielding varieties and more intense cropping are leading to the removal of more sulfur from fields, says Jones.
Sulfur deficiency may be confused with nitrogen deficiency. Sulfur-deficient plants are often stunted with yellowing upper leaves, in contrast to yellowing lower leaves, which are an indication of nitrogen deficiency.
Jones says tissue sampling is necessary because “soil tests for sulfate-sulfur are not a reliable indicator of plant available sulfur."
The uppermost leaves of grasses before heading should contain 0.20-0.25% sulfur, while the top 6” of growth in alfalfa during early bud stage should contain 0.22-0.25% sulfur. Since adequate sulfur and nitrogen are both needed to make protein, the nitrogen-to-sulfur ratio is also a potential indicator of plant sulfur status. Grasses may be sulfur-deficient at a nitrogen-to-sulfur ratio greater than about 15:1, while alfalfa is sulfur-deficient at a ratio above 17:1. However, a good nitrogen-to-sulfur ratio can be misleading if both are deficient.
Sulfur is released from soil organic matter or gypsum (calcium sulfate) at a rate of 4-13 lbs of soluble sulfur per acre per year.
"Deficiency is found more on soils with low organic matter, on side-slope positions and in coarse textured and eroded soils," says Jones.
He adds that sulfur deficiency tends to occur in cool, dry environments and is more common on irrigated coarse soils because of sulfate leaching. Practices that maintain or increase soil organic matter, such as reduced tillage or the addition of manure, can help supply a relatively constant amount of available sulfur.
Growers who suspect sulfur deficiency may want to do a strip trial with readily available sulfur such as ammonium sulfate early next year. If they see a color or height difference compared to an equal strip that only differs by lacking the sulfur addition, a rescue treatment can be applied to the whole field.
He suggests that, if field-response trials or plant tissue analyses indicate deficiency, 20-30 lbs of sulfur per acre can be applied to increase alfalfa and grass yields. Studies in Iowa found that alfalfa yield responses to gypsum applied after the first cutting varied by site. The increases ranged from 0.3 ton/acre with 12 lbs of sulfur/acre to 2 tons/acre with 29 lbs/acre of sulfur. Studies at MSU’s Central Agricultural Research Center at Moccasin found that 25 lbs of sulfur/acre increased forage yield of dryland alfalfa and alfalfa-grass by 30% at one site, but there was no yield increase at two other sites.
Sulfur may not only increase yields; it also influences forage protein. "Because efficient nitrogen conversion into protein requires sufficient sulfur, increased sulfur can lead to increased forage protein content and digestibility and reduced forage nitrate concentrations," Jones says.
He encourages growers to keep records of tissue analysis and protein and/or yield response to strips of sulfur applied to develop their own sulfur management program. In-season applications of readily available sulfur, such as ammonium thiosulfate and ammonium sulfate, can rapidly correct sulfur deficiency. Sulfate fertilizers are not suggested for fall application because they can be lost to overwinter leaching.
Elemental sulfur has the highest sulfur content of sulfur fertilizers (90-100% ); however, it’s slow to supply plant-available sulfur. Therefore, Jones suggests that it be applied in fall or before seeding to give time for some of the sulfur to become available before peak demands. Because of the slow release, elemental sulfur will supply crop needs for a few years.
Annual tissue samples can help track the sulfur status of the field. "If there is adequate sulfur in the soil, levels can be maintained with applications every few years to replace that removed by the harvest," he says.
Alfalfa and grass hay remove 5 and 2 lbs of sulfur/ton, respectively. If the forage is grazed, less sulfur is removed because livestock release most of the ingested sulfur back to the soil.
For more information on forage response to sulfur, see Fertilizer Fact 27 at landresources.montana.edu/FertilizerFacts/ or call 406-994-6033 for a printed copy.