Diagnosing sulphur deficiency in canola

Canola frequently shows early sulphur deficiency symptoms, but yield is often unaffected because plants access available sulphur reserves in the subsoil. Hybrid varieties can display leaf purpling with adequate nutrient levels.

What to look for

Paddock

Smaller sometimes prostrate plants with crisp pale leaves and/or purple leaf undersides that are shown by the leaf rolling inwards.
Symptoms are worse in wet seasons and on soils with low clay, organic carbon or iron and aluminium oxides. Plants may be better on old windrows

Plant

Deficiency symptoms vary between varieties. New varieties are more likely to show chlorotic blotchy 'crisp' leaves in addition to rolled leaves with bright purple undersides that typified old varieties.
Younger leaves are lime-green, often with interveinal chlorotic mottles and pale leaf margins.
Leaves are cupped or roll inwards and become thickened and crisp and brittle. Mildly deficient plants can have subtle purpling on the nearly fully expanded leaf.
Leaves then develop strongly purple undersides that are revealed as leaves roll inwards.
Very deficient plants are can be prostrate; leaves extend further down the petiole than normal, and jut out horizontally rather than forming a domed canopy.
Flowering plants have pale yellow-cream flowers that abort or produce short fat pods with few seeds.

Where does it occur?

Wet conditions

Soil type

High rainfall on sandy acidic soils can result in sulphur leaching
Cold, wet conditions slow sulphur mineralisation and plant uptake

Management strategies

Top dressing

Top-dressing sulphur sources such as gypsum will correct the deficiency
Foliar sprays generally cannot supply enough sulphur to overcome a severe deficiency.
As canola has deep roots there is often no yield increase from applying sulphate to correct an observed sulphur deficiency up to 90 days after seeding if plants can access subsoil sulphur.
Grain sulphur removal (2 to 5, 1.5 kg of sulphur per tonne of canola, cereal grain respectively), is a guide to long term sulphur requirements.
Cost effective sources of nitrogen and sulphur such as sulphate of ammonia should be considered.
Deeper soil testing may eliminate costly sulphur fertiliser application.
Root restricting constraints such as traffic pans, disease or soil acidity will worsen sulphur deficiency and final yield, even if sulphur is available further down the soil profile.

How can it be monitored?

Tissue test

Soil test

As sulphur is immobile in the plant, Young shoot sampling is most accurate. DAFWA trials indicated a critical level of 0.33 per cent S at 90 days after seeding, but plants often outgrew the deficiency at his level, because plant roots accessed sulphur that was deep in the soil profile. Earlier plant tests are less reliable yield response indicators.
Sulphate is readily leached from sandy topsoils but is retained by iron, aluminium and clay in the subsoil.
Trials indicate a 7-8mg/kg 0-10 cm KCl soil test critical limit but this was only accurate in 20 per cent of trials.
Sampling at 30 cm greatly improves soil test reliability. It is also valuable to have subsoil tests of potassium and pH values from the deeper soil profile
Leaf colour symptoms are not a reliable guide for hybrid varieties.

Further information

Plant nutrition [1]