Sulphur deficiency symptoms in canola: what to look for and what to apply

What sulphur deficiency looks like

Sulphur does not move easily inside the plant, so the youngest leaves show it first. DPIRD WA's MyCrop guide describes these signs:

• In the paddock: smaller plants, sometimes lying flat (prostrate), with crisp pale leaves or purple leaf undersides.
• Young leaves turn lime-green, often with yellow mottling between the veins (interveinal chlorosis) and pale leaf edges.
• Leaves cup or roll inwards and become thick, crisp and brittle. Mild cases show faint purpling; worse cases show strongly purple undersides as the leaves roll.
• Very deficient plants lie flat, with leaves sticking out sideways instead of forming a domed canopy.
• Flowers are pale yellow-cream. They may abort, or set short fat pods with few seeds.

What else it could be

• Nitrogen deficiency, phosphorus deficiency and turnip yellows virus can all cause purple leaves. DPIRD says sulphur-deficient plants have rolled leaves with strong purple underneath.
• Manganese toxicity on sour soil (acidic soil) can look similar too: small yellowish plants and cupped leaves. Incitec Pivot reports the two being mistaken for each other in NSW canola.
• A tissue test is the way to tell them apart.

Why canola needs more sulphur than cereals

Soil Quality (a WA partnership including DPIRD and GRDC) puts canola's sulphur need at three to five times that of other crops and pastures, partly because of the sulphur in its glucosinolates (natural sulphur compounds).

• DPIRD gives grain removal of 2–5 kg S per tonne for canola, against about 1.5 kg for cereals. Use this as a guide to long-term needs.
• Soil Quality notes WA growers apply more sulphur to canola than to wheat, yet still less than canola grain removes. Do not count on carryover; test before the next crop.

Where it occurs, and when it costs yield

• Wet seasons, especially high rain on sandy, acidic soils, which can wash sulphate out of the topsoil (leaching).
• Soils low in clay, organic carbon or iron and aluminium oxides.
• Cold, wet weather, which slows the release of sulphur from organic matter and its uptake by the plant.
• Anything that restricts roots, such as traffic pans, disease or soil acidity. These make it worse even when there is sulphur deeper down.

Early symptoms often do not cost yield. DPIRD says canola often shows early sulphur deficiency without losing yield, because its deep roots reach sulphur held in the subsoil. Correcting a visible deficiency up to 90 days after sowing often gave no yield increase where roots could reach that subsoil sulphur. In several NSW DPI trials in northern and central-west NSW, adding sulphur did not change canola yield, oil or protein.

How to confirm and monitor it

• Tissue test: sample young shoots. DPIRD trials found a critical level of 0.33% S at 90 days after sowing, though plants often outgrew deficiency at that level. Earlier tests are less reliable.
• Soil test: a 0–10 cm KCl critical limit of 7–8 mg/kg was right in only 20% of DPIRD trials. Sampling to 30 cm greatly improves reliability, and deeper testing may save an unnecessary sulphur application.
• Test subsoil pH and potassium at the same time. See reading a soil test.

Critical levels vary with region, test method, plant age and yield potential. Always check with your agronomist before acting.

Sulphate or elemental sulphur: what to apply

Plants take up sulphur as sulphate. Elemental sulphur first has to be converted to sulphate by soil bacteria, which needs warm, moist soil, and canola grows through winter.

• Topdressing a sulphur source such as gypsum (calcium sulphate) will correct the deficiency, DPIRD says. Foliar sprays generally cannot supply enough for a severe deficiency.
• DPIRD suggests considering cost-effective N-and-S sources such as sulphate of ammonia: for example Gran-Am, Impact SOA or Agfert SOA. Gypsum products include Calciprill S 14; N-plus-S blends include UreaS and MAXam. Browse all sulphur products.
• Elemental sulphur: Incitec Pivot's use directions for Sulphur Bentonite (90% elemental S) say it must not be used in canola and that sulphur should go on as sulphate. Check the label for the sulphur form of other 90% S products such as Brimstone 90.
• Timing: research cited by Incitec Pivot found sulphate topdressed at the 5–6 leaf rosette, flower buds visible or stem elongation stage gave the same yield and oil as sulphate at sowing.

Sulphate of ammonia also carries nitrogen, so count it in your canola nitrogen budget. See elemental vs sulphate side by side, and urea vs ammonium sulphate for the N side.