Sulfur - how much do you have in the 'bank'?
Key messages:
- canola crops are especially vulnerable to sulfur deficiency
- leaching and high rates of plant removal over the past few years have likely reduced soil sulfur levels
- incremented soil testing to a depth of 60-80cm will identify the quantity and location of sulfur in the soil
- understanding what’s in the ‘bank’ can help farmers make timely and economical fertiliser decisions
Sulfur (S) is an important nutrient for grain crops as it is used in chlorophyll formation and plant development. Canola has a higher requirement than wheat or legume crops for sulfur thanks to its oil and protein production.
Crops get most of their sulfur from soils. However, Australian soils are generally low in sulfur, with organic matter contributing up to 70-90% of soil sulfur. Cropped soils tend to have lower organic matter and therefore lower sulfur.
In the late 1980s, sulfur deficiency was recognised in southern NSW canola crops and this led to a recommendation for the application of 20–30kg S/ha when sowing canola (GRDC Canola guide, 2009).
Given the frequency of the wheat-canola rotation across the Riverine Plains, and our history of regular sulfur applications, how much do farmers really need to apply this year?
The basics
Soil sulfur needs to go through a mineralisation process, similar to nitrogen, in which it is converted to its plant available form, sulfate.
Like nitrate-nitrogen, sulfate-sulfur is soluble and easily moves down the profile. The extremely wet conditions experienced across the Riverine Plains over the past three years has likely resulted in sulfur being lost from soils. In some cases, leaching may have moved sulfur further down the profile, perhaps settling in a layer at depth (stratification), or it may have been leached beyond the root zone entirely.
A combination of leaching and higher-than-average canola yields over the past three seasons may have depleted soil reserves, with crops at risk of deficiency if maintenance programs haven’t kept up with system losses.
What does deficiency look like and how is it fixed?
Sulfur deficiency can show as pale and mottled leaves from early rosette through to stem elongation in canola. In severe cases, leaves may also be cupped with a purple margin. At flowering, flowers may be pale yellow to cream, with poor podset. Sulfur deficiency can result in significant yield loss, even if symptoms aren’t visibly severe.
Sulfur deficiency can sometimes be confused with nitrogen deficiency, however yellowing occurs in younger leaves instead of the older leaves.
The good news is that sulfur deficiency can quickly be corrected in-crop using mineral sulfate fertilisers, such as sulfate of ammonia, which can recover yield and oil potential if applied before stem elongation.
While it’s important not to underestimate crop requirements, there is still conjecture about how much sulfur is removed by canola. In the past, it was generally accepted that canola needs about 10 kg of sulphate-sulfur per tonne of grain produced, but recent research from the GRDC northern zone suggests that the actual figure could be around half this, meaning maintenance levels could be more like 4-5 kg S per tonne of grain removed. The currently accepted critical value for sulfur at 0-60cm is 31 mg/kg (KCl-40 S+ test), with results below this indicating a potential deficiency.
In canola paddocks with a history of sulfur application, even if it hasn’t been applied recently, there may not necessarily be an economic response to sulfur applications due to high levels of residual sulfur in the subsoil.
But the only way to be sure is to soil test.
Soil testing
Sulfate is highly mobile in the soil and for this reason 0-10cm soil tests are not suitable for diagnosing deficiency. Instead, aim to sample to a depth of 60-80cm and segment tests into 0-30cm and 30-60cm increments (at a minimum). This will provide more information about how much sulfur is in the soil and where it is located. Properly interpreted, the soil test results will also identify any deficiency and how quickly, and at what rate, sulfur will need to be applied.
Growers can piggyback sulfur testing onto deep soil nitrogen (DSN) testing programs, while tissue testing is another option.
Nitrogen and sulfur
Variable responses to sulfur have been observed depending on background nutrition levels in canola. In some cases, this is due to the suboptimal supply of nitrogen, which can inhibit the uptake of sulfur by plants.
During 2017-18, Riverine Plains and GRDC investigated nitrogen supply and sulfur uptake through the Optimising crop nutrition in canola project. The project reinforced basic sulfur nutrition to ensure nitrogen supply is not limited and highlighted the importance of using soil tests to inform sulfur application rates, similar to the way that DSN testing informs urea application rates (rather than relying on standard sulfur application rates with every canola crop).
Summary
Now is the time to manage sulfur deficiency in soils.
If symptoms become visible in canola, or if soil tests indicate a potential deficiency, talk to your agronomist for further information on tissue testing and fertiliser application strategies.
Resources and further reading
Potassium and sulfur – how much is enough
Incitec Pivot - insights into sulfur in canola
Sulfur strategy to improve canola profitability in Central-West NSW
