Date

2023/08/31

Duration

5 min read

ebook

Soil Quality: 10 Plant Nutrition

Organisations

SoilsWest

Department of Primary Industries and Regional Development

Grains Research and Development Corporation

Murdoch University

Authors

Geoff Anderson

The spread of canola as a profitable break crop in the south-western agricultural region of Western Australia has bought a new focus on sulfur nutrition in farming systems. The increase in the area sown to canola has been supported by continuous improvements in grain yield (0.087 t/ha/year), effective control of grass weeds and and greater disease (Blackleg) resistance in new canola varieties.

Canola has a three to five times higher sulfur requirement than other crops and pastures. The higher sulfur requirement of canola is in part due to the sulfur contained in glucosinolate compounds. Older rapeseed varieties that contained higher levels of glucosinolates than the modern canola varieties and had even higher sulfur requirements.

In new canola varieties, typical symptoms of sulfur deficiency are chlorotic blotchy ‘crisp’ rolled leaves (in old varieties, rolled leaves with bright purple undersides were more typical). Young leaves are often lime-green, with interveinal chlorotic mottles and pale leaf margins. Older leaves develop intensely purple undersides and leaves that roll inwards. Flowering plants have pale yellow-cream flowers that abort or produce short fat pods with few seeds.

Sulfur deficiency in young leaves (A) and flowers (B) Note sulfur-deficient flower on the right with pale yellow petals.

Soil test critical levels are higher for canola than for wheat but similar to those for pastures. For canola, soil sampling to a depth of 30 cm was found to better define critical ranges than 0-10 cm depths.

Critical levels and range of sulfur in soil for pasture, wheat, and canola

Crop Sampling depth Critical level (mg/kg) Critical range (mg/kg)
Pasture 0-10cm 8.0 6.0-10.0
Wheat 0-10cm 4.5 3.5-9.5
Canola 0-10 6.8 6.0-7.7
0-30 7.1 6.7-7.5

The higher sulfur requirement of canola is reflected in higher tissue sulfur critical levels than for wheat and lupins. However, critical levels can change with plant age. Therefore, it is essential to note plant age when taking plant tissue samples to diagnose sulfur deficiency.

Due to its higher requirements, application of sulfur fertiliser by growers is generally higher for canola (12.3 kg S/ha) than for wheat or lupin (5.9–6.6 kg S/ha). At these fertiliser rates, the sulfur applied is greater than sulfur removed by grain for wheat, lupin, and pasture but less for canola. Sulfur removed in canola grain is in the order of 3.8–4.0 kilograms of sulfur per tonne of seed compared to wheat (0.8–1.4 kilograms of sulfur per tonne of seed) and lupin (1.7–2.9 kilograms of sulfur per tonne of seed).

Soil mineralisation, re-cycling of crop residues, and leaching are the critical soil processes determining the levels of plant available sulfur during the growing season. In the south-western agricultural region, the low water holding and sulfur adsorption capacity, combined with intense June and July rainfall, can result in significant sulfate leaching and the appearance of sulfur deficiency in canola. Even with higher sulfur fertiliser inputs for canola, the greater removal in seed and the risk of leaching losses mean that growers should not assume there is a significant carryover of soil sulfur after canola for following crops in the rotation. Soil testing can be used to guide decisions about sulfur fertiliser rates to apply to crops and pastures following canola.

Critical levels of sulfur in plant tissue for canola, wheat and lupin

Crop Plant part Plant age (days post-germination) Critical value or range (%)
Canola Whole shoots 90 0.40
Seed 0.36
Wheat Young leaves 21 and 44 0.31 and 0.14
Whole shoots 21 and 44 0.29 and 0.11
Lupin Young leaves 44 0.07
Stems 44 0.10
Whole shoots 44 0.16

References

ebook Soil Quality: 10 Plant Nutrition

Scanlan C, Bell R, Weaver D, Borrett R and Cheng M (2023).

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