Focusing on the ratio of nitrogen to sulphur in growing grass can help producers increase not just yields but also improve sugar and protein contents, latest research suggested.

Identifying the precise amount of sulphur in grass through simple laboratory analysis and relating this to the total nitrogen content of the plant can open the doorway to more accurate fertiliser applications and improved nitrogen fertiliser use efficiency (NfUE).

Such an approach can easily result in a return of £5 to £15 for every £1 spent on putting sulphur into you fertiliser programme, according to Dr George Fisher, of CF Fertilisers. “Sulphur is just as important for grass growth and quality as Nitrogen, Phosphate and Potash,” he explained.

“Like nitrogen, sulphur is essential for grass protein production and is at the core of two of the 12 essential amino acids that makes up these proteins.

“If you don’t have enough sulphur available for plants to produce the required proteins, then yield drops, but as proteins are involved in all plant processes, this also effects crude protein levels and sugar contents. So not only do you end up with a reduced volume of grass, it’s also of significantly lower quality.”

Grass requires almost half as much sulphur as nitrogen but although much of this used to be available as atmospheric deposition from industry, this is no longer the case, he explains.

In fact, by 2015, emissions of sulphur dioxide had fallen by 96% compared to 1970 and whereas 70-80kg/ha of the optimum 100kg/ha requirement came from such depositions, it was less than 5kg/ha by the 2000s and is even less now.

“As time passes by, we are effectively mining the remaining S that is left in the soil which is resulting not only in lighter soils becoming deficient, but more recently medium soils and even grass on heavier clay loams are showing response to applications. The undeniable fact is the vast majority of grass production areas in Britain are now S-deficient,” he pointed out.

Measuring sulphur in soil is at best unreliable and variable, so it is much better to test the growing grass itself. “This will give the best picture of whether you will benefit from S-containing fertilisers and point the way to the most appropriate application options. The amount of S in grass is normally in the range 0.2-0.4%, with at least 0.25% being needed to avoid the effects of deficiency,” said Dr Fisher.

“Whilst knowing the actual amount of S in grass is a good starting point, what is far more informative is understanding this relative to the amount of N there is in the sample, so you need to make sure that any test you undertake can provide this.

“A ratio below 10:1 shows sulphur is OK. Anything above 10:1, with an S content of less than 0.2%, is deficient. It’s very easy to sample growing grass, so get an analysis done and make a decision.” (See table 1)

If you need S then a good rule of thumb is to apply 50-100kg SO3/ha in total over the season for grazing and 30-50kg SO3/ha for each silage cut is a good target, he pointed out.

“Because sulphate can move through the soil profile, the general approach should be to apply sulphur ‘little and often’ to maximise efficiency and optimise the effects.

“This is why it is best to apply S with every nitrogen fertiliser application and why NS, NKS and NPKS true granular compounds are so effective, providing a range of opportunities to optimise N and S ratios whilst addressing other potential nutrient needs.

“Responses to S can vary for a number of reasons including soil type, season and winter rainfall so where possible it’s a good idea to talk to a FACTS qualified advisor about your individual situation. Some situations could respond to higher levels of S and provide greater economic returns so it’s well worth learning as much as possible about your needs.”

Get the balance right and trials show you can expect to see grass dry matter yields increased by up to 35% for silage and 20% for grazing. “Replicated trials over the three years ending in 2018 on medium loam soils in Cheshire using S-containing CF SingleTop (27-0-0+12 SO3) in preference to straight ammonium nitrate, increased silage yields by 7t DM/ha and in grazing by 3.4t DM/ha.

“If that extra grass energy is used to replace concentrate feed, the return on investment (RoI) for using the SingleTop for silage would be 8:1 and in grazing it would be 5:1.”

But such benefits do not factor in the effect on grass quality, where trials have also show significant gains, Dr Fisher pointed out. “Reduced quality can affect your herd in numerous ways, not only in terms of the direct response to reduced energy and protein content, but also through reduced palatability and dry matter intake.

“Independent trials at Rothamsted Research (formerly IGER North Wyke) have shown average lifts in crude protein of 2.3% for silage and up to 0.6% in grass sugar content – or 3. 5%, compared to 2.9%.

“Making more milk or meat captures more value than replacing concentrates, for example, but generally you can expect a £5 to £15 return for every £1 you spend on well managed sulphur applications.”

One of the most important aspects of ensuring the impact of fertilisers on the environment is minimised is to manage them in such a way that as much of the N applied is used by the plant and not lost from the production system, Dr Fisher explained.

“This is where sulphur has such a vital role to play as it can help boost N use efficiency significantly with trials showing an increase in NfUE of at least 10%. Furthermore, N losses from soil have been shown to be reduced by 57% over winter on grassland using S fertilisers, compared to those without.

“Because sulphur and nitrogen are so integral to protein production, a lack of sulphur leaves more nitrogen unused by plant growth and left in the soil from which it can potentially be lost.

"Evidence is now mounting that NfUE and actual leaching losses are reduced where S is included in the nutrient plan, regardless of region and soil type and this can only have a beneficial effect on water quality overall,” argued Dr Fisher. (See Table below)

Table 1

Diagnosing sulphur deficiency with grass N:S ratios

N:S Ratio Grass S less than 0.25% Grass S more than 0.25%

Over 13.1 S-deficient S-deficient

10:1 to 13:1 S-deficient Grass OK

Below 10:1 Grass needs more N Grass OK

Table 2

Nitrogen fertiliser use efficiency (NfUE) with sulphur

Fertiliser NfUE on SILAGE (%) NfUE on GRAZING (%)

AN 65 74

SingleTop 79 84