Sulphur is an essential plant nutrient required for the production of amino acids which in turn make up proteins. In cereals lower sulphur levels lead to lower protein and given this affects the quality of the flour, the price received for this grain will be reduced. Lack of sulphur will also affect the oil content and hence the price received for canola.
Yield losses also occur in low sulphur situations, especially with canola. Ideally, plants will take up sulphur at the same levels as phosphorus, that is, it is a major plant nutrient.
Role of Sulphur in Plants
Along with nitrogen, sulphur is a critical component of the amino acids which build proteins within the plant. Sulphur is present with varying degrees in nearly all soils. Soils with clay and gravel have generally more sulphur present than sandier soils from high rainfall areas. This is due in part to the composition of the original parent rock. Organic sulphur, which is mineralised into plant available sulphate sulphur, is more prevalent in soils with high clay and gravel content. The sandier soils from higher rainfall areas do not have any ability to restrict the leaching of water soluble sulphate sulphur.
Sulphur remaining in palnt residues is readily recycled into the soil (see Nutrient Removal).
Sulphur deficiency symptoms in cereals will often, from a distance, resemble nitrogen deficiency, in that plants will have pale green to yellow leaves present. Closer inspection will reveal that with sulphur deficient plants the yellow leaves will be the youngest leaves, unlike nitrogen deficiency which affects the oldest leaves. Similar symptoms are present in clover plants in that yellow leaves are present and when the deficiency becomes severe, the leaves will stand upright with a ‘cupped’ appearance. Canola plants, which have a high requirement for sulphur, have slightly different symptoms of deficiency. The leaves will ‘curl’ and whilst the leaves will turn yellow the veins will show a marked red to purple colouration.
Modern high analysis fertilizers will usually contain enough sulphur to supply sufficient levels to cereal crops. Canola, however, will require more than can be safely or conveniently applied using a seeding fertilizer and so extra sulphur must be applied, either before seeding as gypsum, or post seeding as Amsul, (sulphate of ammonia).
If a deficiency manifests in an established crop, this can be easily corrected with an application of sulphate of ammonia.
Supplies of Sulphur (Elemental or Sulphate)
Plants take up sulphur in the sulphate (SO4) form. The sulphate form is water soluble, and being an anion, is readily leachable. The elemental form of sulphur needs to be broken down into the sulphate form before becoming available to the plant. This is achieved by a bacteria (thiobacillus) which digests the sulphur and excretes sulphate. All soils contain this bacteria. It takes about a fortnight for elemental sulphur to start breaking down, so it should be used before a plant deficiency can be seen. In waterlogged conditions, where sulphate sulphur will be lost by leaching or runoff, the bacteria will become dormant, so sulphur will not be lost.
Pros and cons of the two sulphur sources.
- Immediately available to the plant
- Water soluble
- Quick acting
- Can be lost with one heavy rainfall event
- Sustained release
- Not lost by leaching
- More available when maximum plant growth occurs in spring
- Will build up a sulphur "bank"
- Slow to break down
- Not suitable to correct a visual deficiency in plants