Crop yields are being held back by a lack of sulphur with just under half of the UK cereal area and a quarter of the oilseed rape area not receiving any inorganic sulphur fertiliser in 2014.
Yet growers cannot afford to miss out on the returns through increased yield and quality, which far outweigh its cost of about 20p/kg.
According to Agrii’s Northern R&D manager, Jim Carswell, these Defra fertiliser use survey figures are astounding and suggest that the messages on the importance of applying sulphur have not yet reached every grower.
“All the evidence from trials and research is showing that crops need sulphur,” he says.
“And at the same the time the message about lowered sulphur deposition has been going on for years.”
So why are some growers not taking the sulphur message on board?
The answer could be that growers don’t know their crops are at risk from deficiency, nor the associated impact on yield and quality (see “Role of sulphur” below).
Sulphur deposition from the atmosphere is now largely discounted as having any real value for agriculture.
So unless growers can accurately quantify the available sulphur from organic sources, or are satisfied their soils are supplying sufficient, available amounts, then inorganic sulphur (as sulphate) should be routinely applied.
An added difficulty is that symptoms can be subclinical in cereals or the pale leaves confused with nitrogen deficiency.
In oilseed rape, pale leaves and flowers are indicative of low sulphur in the crop, but once this is visible there may be insufficient time to rectify it.
But what is clear is that if you don’t measure it, you can’t manage it, says Mr Carswell.
Role of sulphur
Sulphur is a key part of protein synthesis, being a part of the amino acids cysteine and methionine.
Therefore, an insufficient supply will result in a decrease in the supply of these amino acids for protein synthesis, thereby limiting yields.
Deficiencies in milling wheat crops affect the baking quality because the sulphur amino acids in the gluten fraction of flour are responsible for the elasticity of the dough and bread volume.
In spring barley, yield increases help to dilute the grain N and improve the malting quality and the flavour of beer, while in oilseed rape, sulphur improves yields and minimises the occurrence of green seeds.
Which is why he suggests growers should build up a picture of their sulphur status over a season by utilising soil, leaf and grain analysis to evaluate levels.
“Then you can make a more educated decision on how to manage sulphur levels in crops on a field-by-field basis.”
Crops that growers should be concentrating on are grass, oilseed rape and cereals, especially milling wheat and malting barley.
Soils differ in their sulphur availability, with lighter soils more likely to be deficient due to low organic matter and higher potential for leaching. But soil analysis is not a good indicator of sulphur soil status, says Mr Carswell.
Instead, he suggests growers use the malate:sulphate leaf tissue test and a grain N:S ratio test.
The first gives a snapshot in season and the latter, although not predictive, allows growers to see the historic success of nitrogen and sulphur management.
Mr Carswell has taken this approach on the Agrii research sites to check winter wheat crops are receiving sufficient sulphur in the spring.
Allison Grundy of CF Fertilisers UK (formally GrowHow) believes the malate:sulphate test is the most accurate.
The test involves sampling the youngest, newly emerged leaves in oilseed rape and wheat as soon as the crop is beginning to grow in the spring. If the ratio is above 1.5:1 then the crop is deficient.
This test gives a grower enough time to apply sulphur and rectify the deficiency.
But there are some caveats and the time of sampling is critical.
“If the crop is not actively growing the test may not pick up a deficiency, or the crop may be in a transient phase where its root system is not big enough and cannot access soil sulphur.”
So while testing can be useful, she believes that on oilseed rape, sulphur applications should be routine.
“Growers need to find an excuse not to apply it rather than one to apply it,” she says.
On a forward crop with a green area index (GAI) greater than 1, Ms Grundy recommends applying a small amount of nitrogen plus sulphur in late February and then the balance in mid-March.
But on more normal crops with a GAI of 0.75-1.0, sulphur can be applied in one go at the first nitrogen timing dose. Recommended rates are 80-100kg of sulphate, irrespective of the yield potential of the crop.
On milling wheat, 40-50kg of sulphate should be applied at either the first or second nitrogen timing or applied little and often – at every N application if you have a product which contains a smaller concentration of sulphur such as SingleTop (27N, 12SO3).
“This usually ends up applying more sulphur and it can be a good approach if you are growing on very light soils.”
For spring barley, sulphur can either be applied in the seed-bed or at the two- to three-leaf stage depending on the amount of planned nitrogen.
|Total sulphur £16/sample **
Available sulphur £10/sample **
|Less than 10ppm sulphur*
|Sample in spring.
Note – due to mobility of S in the soil, where soil N-Min tests are low, S is likely to be low too.
|£15.75/test***||Ratio >1.5 in wheat and OSR and >3 in barley||Sample after the end of Feb – GS 1.10||Test after early April – GS22-27||Test after early April – GS22-27|
|Grain N:S ratio
|Grain or seed £16/sample**||Ratio >15:1||Test seeds/grain after harvest to check back on the season’s S management|
*ADHD project report 374