Lower emissions from powerstations and industrial processes are reducing the amount of sulphur deposited in soils, forcing growers to rely increasingly on artificial alternatives.
Faster growing and higher-yielding varieties are also having an impact as they have greater sulphur demands. But figures suggest some growers have not responded to this change, with many applying too little or none at all.
Research carried out by Growhow between 1999 and 2005 showed just 60% of growers were using sulphur on oilseed rape and 40% on cereals.
Oilseed rape has the greatest demand for sulphur of any combinable crop and on deficient soils yields can be halved if no artificial product is applied, says David Parish, agronomist for NIAB TAG. “Most growers will have a 95% chance of getting a return on their investment if they apply sulphur to oilseed rape.”
This season is the first time he has seen sulphur deficiency in the autumn, suggesting the problem could be getting worse. “The general reduction in atmospheric pollutants, including sulphur is well known. But it’s possible higher than normal rainfall in many areas during August and September has also played a part, by carrying nutrients beyond the rooting zone.”
Artificial suphur is, therefore, crucial and all crops should receive between 60-100kg/ha of SO3. Crops showing signs of deficiency in the autumn should be at the higher end of the spectrum, but those on heavier soils might get away with a 60kg/ha application, he says.
Testing for sulphur deficiency is not an exact science and growers need to build up a picture over time of the state of their soils, he says. “If any crops show signs of deficiency it is a good indication that soils are not providing enough.”
Sending leaves for lab testing can give a more accurate indication of the rates needed in the current and proceeding crop, he adds.
Mr Parish reckons early March is the best time to apply bagged sulphur products. “It needs to be available to the crop at stem extension (mid March) and ideally it wants to go on slightly earlier so it will be available to the plant.”
But because SO3 is highly leachable, it shouldn’t be applied too early, he warns. “If you get heavy rain after application it can be washed away before the plant is ready to use it.
Growers who have ordered ammonium sulphate fertiliser should be able to apply the full sulphur dose in one hit, but those with higher nitrogen products may end up applying the sulphur component in splits, he explains.
“The danger is that these products may be applied according to nitrogen timings, so you need to make sure you are getting enough sulphur on early.”
Light, sandy soils are most at risk from sulphur deficiency due to their lower nutrient holding capacity and yields could be halved if it isn’t managed effectively in the spring, says Mr Parish.
Heavier soils tend to deliver more sulphur to the crop, but can also suffer sulphur deficiency if the root path is impeded by compacted soils, he says.
“I’ve seen sulphur deficiency in crops planted on heavy ground, which would normally be low risk, due purely to poor soil condition.”
Any foliar sulphur holding sprays applied in the autumn should be discounted and not be used as a reason for reducing sulphur rates, he says “Autumn sulphur sprays only alleviate the problem in the short term and hold the situation until spring.”
Russell Davison, fertiliser manager for Openfield, says sulphur deficiency in oilseed rape crops last spring were the worst he has seen for a number of years.
He also believes better establishment techniques combined with the use of higher yielding and faster growing hybrid varieties are partly to blame. “These more voracious, vigorously growing crops do have a higher nutrient appetite and it appears that in some instances sulphur supplies simply ran out.”
Openfield’s figures suggest average oilseed rape yields last harvest were 10% higher than the previous year at 3.6t/ha, meaning crops’ nutrient demands are steadily increasing, he says.
“With crops well set this autumn and atmospheric deposition now negligible, we expect high sulphur demands again this spring.”
Growers should look out for pale yellowing of leaves and slower plant growth, but remember that symptoms can be mistaken for lack of available nitrogen or other deficiencies, says Mr Russell.
If sulphur deficiency is not spotted until flowering when there will be a reduced number of pale flowers, 60% of the crop’s yield potential can have been lost, says Mr Davison.
Even low-level sulphur deficiencies with no visual symptoms can cut yields by 10-20%, so it’s important to get it right, he warns.
Mr Davison reckons most oilseed rape crops require about 100kg/ha SO3 every season. On heavier land, one early application could suffice, but two to three splits may be more appropriate on lighter land to minimise leaching losses, he says.
Balancing the right nutrient ratio is crucial when choosing products to minimise the number of applications needed and reduce over application, he adds.
“Growers need to realise sulphur is far too important a nutrient to be left until the last moment and they should be building it into their strategies sooner rather than later.”
Sulphur demand in cereals is roughly half that of oilseed rape, but there is an increasing demand for artificial inputs, particularly on lighter soils, says Mr Parish.
Typically cereals should receive 40-60kg of SO3, but growers should expect yields responses to be considerably lower than oilseed rape. However, these could be up to 10% on lighter soils.
Impeded rooting also compounds the problem in cereals and he has seen cases of sulphur deficiency in second wheat crops on heavy ground.
Growers who suspect sulphur deficiency in cereals should react quickly and not assume the crop is short of nitrogen.
“In cereals, the crop just looks a bit off colour, which is often blamed on a lack of nitrogen.”
Applying nitrogen to a sulphur deficient crop is the worst course of action as it further restricts sulphur uptake.
“If you are unsure, you should carry out a leaf test. Take the youngest, fully-expanded leaves and send them to the lab.”
The result can help rectify the problem in the current crop, but most importantly help build up a picture of the sulphur levels in the soil.
Some growers on heavy land are still able to grow a cereal crop without applying sulphur, but more research is needed to determine the amount of sulphur offered by different soil types, he says.
Allison Grundy, agronomist for Growhow, says sulphur is particularly important for protein quality and development in milling wheat. “Wheat grown under sulphur deficient conditions is unlikely to satisfy millers’ requirements.”
This is because sulphur plays an important role in converting nitrogen into protein and helps give the resulting flour elasticity, she says. “There is also a clear relationship between sulphur content and loaf volume.”
Spotting sulphur deficiencyOilseed rape
Yellowing between leaf veins – confirm with laboratory test.Cereals
Pale colour, easily mistaken for nitrogen deficiency – confirm with laboratory test
Most research on sulphur was carried out in the 1980s and 1990s and there is very little new information, says Allison Grundy, agronomist for Growhow.
“We know deposition has decreased since then, but we don’t know how much this has affected sulphur response on different soil types.”
The try and find out the firm has commissioned a three-year sulphur response trial with ADAS over three sites, which starts this spring.
One site is on light soil and is known to have a sulphur deficiency, while the other two are on heavier ground, says Ms Grundy.
“There will always be a greater response on lighter soils, but we want to see if the effect of sulphur application on heavier soils has increased.”
Case study: Richard Monk, Hampshire
Correct nutrient application including sulphur means Richard Monk from Rookley Farm, near Stockbridge, Hampshire, is able to return wheat yields of 10t/ha on less than 30cm of soil.
His soils are mainly light loam over chalk so making sure crops are not short of sulphur is essential. Mr Monk applies 80-100kg/ha of SO3 on oilseed rape and 40-50kg/ha on cereals. “It generally goes on with the first dose of nitrogen, so on oilseed rape it is applied in late February and cereals in early March.
Double Top is the main product used, so most oilseed rape recieves roughly 80Kg/ha nitrogen and cereals about 40kg/ha, he notes.
As sulphur is only available to the current crop, he doesn’t drop application rates when cereal prices are down or fertiliser prices are up. “You need to take a longer term view on pricing and apply the product every year.”