Application methods, placement and timing depend on soil conditions in each field
Your crops will never run short of sulfur, a crucial building block of healthy plants, and you won’t waste fertilizer dollars if you follow The Fertilizer Institute’s 4Rs of Nutrient Stewardship. The 4Rs include applying the right fertilizer source at the right rate at the right time in the right place.
Of course, you won’t need to apply sulfur fertilizer if your soil already is supplying a sufficient amount and if it’s being taken up by plants, says Farm Journal Field Agronomist Ken Ferrie. Sulfur management should be based on soil testing and plant tissue analysis.
First, scout your corn fields for symptoms of sulfur deficiency. "Visual symptoms tell you to be suspicious," Ferrie says. "They include pale color, stunted growth and delayed maturity. Sulfur deficiency symptoms are similar to those for nitrogen deficiency.
"The difference is that a nitrogen deficiency shows up in the older leaves at the bottom of the plant. Sulfur deficiency shows up in new growth—the top of the plant or the whorl of corn plants. That’s because plants can move nitrogen from the old growth to the new growth, but sulfur, unlike nitrogen, is not mobile."
Be alert for sulfur deficiency on fields that have been heavily manured. "Soil phosphorus levels above 90 ppm [parts per million], often resulting from high rates of livestock manure, inhibit sulfur uptake by plants," Ferrie says.
Tissue and soil tests. If you suspect a sulfur deficiency, confirm your observations with tissue analysis and soil tests. Have plant tissue analyzed several times during the season—early, mid-season and late. To fertilize effectively, you need to know when plants are running short of sulfur, and then time applications accordingly.
Test your soil to see if there’s a sufficient supply. "Look for high, medium or low sulfur levels," Ferrie says. "Keep in mind that different laboratories use different methods to extract sulfur and have different ways of reporting the results. One lab might consider a reading of 8 ppm to 12 ppm to be medium, while another lab’s idea of medium might be 25 ppm. Don’t compare results from one laboratory to those from another unless you know both labs used the same extraction process. You can avoid confusion by staying with the same laboratory for all of your soil testing."
If your soil test reveals adequate levels of sulfur but tissue tests show a deficiency, the sulfur in the soil is not getting into the plants. There could be several causes.
"Soil compaction is the first thing to look for," Ferrie says. "Other possibilities include saturated soils and cold soils, both of which impact microbial activity, and severe root pruning by insects such as rootworms. In those situations, the solutions include reduced tillage, improved drainage or pest management to enable plants to utilize the sulfur in the soil."
Soil acidity (discussed below) can also impede sulfur uptake.
The right product (sulfate versus elemental sulfur) and the correct timing, placement and rate will vary depending on the situation. "With sulfur, there’s no one-size-fits-all method of treatment," Ferrie says.
Sulfur is not mobile in the plant, so early uptake is vital to prevent stunting and shortened internodes.
Pay the carbon penalty. If a sulfur deficiency shows up shortly after emergence, the cause might be the carbon penalty. The carbon penalty occurs when soil sulfur levels are adequate, but there is an abundant supply of old crop residue, as with continuous corn. With all that food available, microbial populations explode. Because the microbes use sulfur, they temporarily immobilize the supply in the soil. That sulfur becomes available again later, after the microbes die and decompose, but the plants experience a deficiency through six or seven growth stages.
Sulfur deficiencies early in the growing season are especially troublesome. "Plants need sulfur throughout the vegetative growth stages. Since sulfur is not mobile in the plant, early uptake is crucial. A deficiency at that time will cause stunting and shortened internodes. So be aggressive about correcting early-season deficiencies," Ferrie says.
In this situation, when a grower has adequate soil sulfur levels, he could broadcast a small amount of sulfur in the readily available sulfate form in the spring. "But adding sulfur to your starter fertilizer and banding it 2"x2" from the plant probably is twice as effective," Ferrie says. "Banding the fertilizer near the roots ensures that it won’t be tied up by soil microbes and there will be no soil temperature issues."
Low-organic-matter soil. Different timing is required with low-organic-matter soils. "The lower the organic matter content, the less sulfur is provided by the soil," Ferrie says.
"With low organic matter content and low sulfur levels, you’ll probably have deficiencies all season long—early, mid-season and late. In this situation, you want to raise the sulfur level in the soil. So broadcast some sulfur fertilizer at a higher rate. If possible, also apply some in a band to make it easy for roots to reach it. If you use the elemental form of sulfur, apply it in the fall so it has time to convert to the sulfate form by spring. The sulfate form can be applied either in fall or spring.
"Another option is to spring-apply a mixture of the elemental and sulfate forms. Because the elemental form takes some time to oxidize into sulfate, it will become available to plants later in the season."
Remember, sulfur can be leached out of soil by water, just like nitrogen. "The lower the water-holding capacity or cation exchange capacity of a soil—in other words, sandy soil—the higher the risk of leaching," Ferrie says. "On these soils, don’t apply the sulfate form of sulfur in the fall; apply elemental sulfur instead. If you apply sulfur in the spring on sandy soil, use a mixture of sulfate and elemental sulfur.
"On irrigated sandy soil, you can apply sulfur through the pivot throughout the growing season. On non-irrigated sandy soil, consider splitting your sulfur application by broadcasting some early and applying ammonium sulfate in your sidedress fertilizer."
Acid and alkaline soil. Any field with acid soil will have poor sulfur uptake because acidity ties up sulfur and makes it unavailable. The best solution is to balance the soil pH as quickly as you can. "Until you correct the soil acidity, you may have to band, sidedress or foliar-feed sulfur fertilizer to override the acidity," Ferrie says.
Acidity is common on muck or organic soils, Ferrie adds. "It is very difficult to change sulfur levels in that kind of soil. On those soils, band an application of sulfate fertilizer with your planter. If you irrigate, follow up with multiple applications of low rates of sulfate through your center-pivot system. If you don’t irrigate, you can foliar feed sulfate. The plants’ new growth will turn green after application, but the later growth will continue to be sulfur deficient, so you may need to make multiple applications."
Alkaline soils can have sulfur uptake problems as well because sulfur availability is limited in high-pH conditions. "Rather than trying to build up soil sulfur levels, make an annual application to maintain the crop," Ferrie advises.
Other considerations. Soil acidity can be a byproduct of sulfur application. "Although sulfate is not acidic, the ammonium in ammonium sulfate is twice as acidic as the ammonia form of nitrogen," Ferrie says. "Every pound of ammonium sulfate requires about 8 lb. of limestone to neutralize the acidity.
"Any soil acidity resulting from sulfur fertilizer application will be accounted for in your routine soil test," Ferrie continues. "Acidity usually only becomes an issue when growers apply large amounts of ammonium sulfate for the nitrogen, rather than for sulfur."
Sulfur’s leaching potential is why the right rate is so important. Wasting fertilizer is bad enough, but the escaping sulfate takes other nutrients with it. "Sulfate ions bond with potassium, magnesium and calcium," Ferrie says. "What leaches out is potassium sulfate, magnesium sulfate and calcium sulfate."
That usually only happens with large ammonium sulfate applications over time. "But in one such area, some farmers reduced their soil magnesium levels by 50% over 10 years," Ferrie says. "In this situation, farmers may be forced to apply dolomitic limestone or K-Mag to rebuild magnesium levels."
Sulfur needs more attention because the amount available free in the atmosphere has declined. If you use soil and tissue testing and abide by the 4Rs, your crops will never go hungry for this vital micronutrient.
Gypsum: A Sulfur Source
Gypsum (calcium sulfate) can be part of your sulfur fertility program. "It’s ideal for situations where you need sulfur for plant health, and you also need calcium to flocculate clay particles and improve structure," says Farm Journal Field Agronomist Ken Ferrie. "Gypsum has the advantage of not being acidic like ammonium sulfate.
"However, the calcium in gypsum does not neutralize soil acidity. Gypsum is a source of sulfur but not a source of lime," Ferrie says. "If your goal is to lower soil acidity, apply calcium carbonate instead. The carbonate will reduce acidity, and calcium will flocculate soil."
Learn and Profit from Nutrient Navigator
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher. www.FarmJournal.com/nutrient_navigator
You can email Darrell Smith at firstname.lastname@example.org.