Do I need sulphur with my nitrogen?

It is unlikely that you will get a sulphur response over winter from autumn-applied S, since growth rates are low meaning demand for S is very low. Additionally, the clover plants which will likely have the greatest response to the S inputs are dormant over the winter months.

Sulphur with nitrogen for spring applications
A sulphur-limited N response is much more likely to occur in spring. This is as a result of sulphate leaching over winter, combined with cool soil conditions under which the rate of organic S mineralisation is very low. This may result in pastures being temporarily deficient in sulphur at this time, as plant demand for sulphur outstrips soil supply. If soil sulphur levels are too low (i.e. deficient relative to the crop/pasture requirement), this may limit the efficiency at which the pasture responds to fertiliser N, and could also limit clover growth as temperatures rise.

Managing sulphur inputs
If fertiliser plans allow for spring application of sulphur, then apply sulphate-sulphur then to get the best S response. Applying sulphate sulphur in autumn is inefficient given the pasture is unlikely to respond to the sulphur at that time and leaching losses mean that the sulphate may not be there in the spring when is it taken up by the actively growing pasture.

Remember that you only need 30-50 kg S/ha to overcome a strong sulphur deficiency (range is dependent on soil type – see the FMRA handbooks). So if farmers are applying maintenance PKS later in spring, only relatively small amounts of S will be needed to ensure there is sufficient S available for the early spring period.

In spring, herbage testing should be used to complement soil testing when deciding on whether just straight N or an N+S product such as n-rich ammo is required. Below is an interpretation guide for mixed pasture herbage samples in spring: S levels between 0.26-0.3% S - S may limit an N response 

  • S levels at or above 0.3% S - S will not be limiting an N response
  • S levels between 0.26-0.3% S - S may limit an N response
  • S levels less than 0.26% S - S is likely to limit an N response

Pasture plants contain N and S at around 4-5% N and 0.3-0.4% S, respectively – approximately a 12:1 ratio for N and S; so plant uptake / demand is also in this ratio.Superten). So, to make sure that SOA stocks are used appropriately, do not apply SOA as a straight line product, it should be used in a blend such as in the Ammo product range.

Comparatively, SOA at 20.5% N and 23% S, has a N:S ratio of around 1:1, making the amount of S in this product an overkill for most situations when other ‘maintenance’ S will be applied separately (e.g.

New product
A new product formulation, n-rich ammo 40N, is being created as a 25% SOA and 75% urea blend. This provides a product with 39.6% N and 5.8% S, giving an N:S ratio that is more closely aligned to that which is required by pasture.

Sulphur with nitrogen for autumn and early winter applications
Pastures are not likely to give additional response to sulphate of ammonia or N-rich ammo, over and above urea or sustain for autumn and early winter applications. The reason for this is that organic-S mineralisation over summer and autumn means that nitrogen responses are not likely to be sulphur-limited going into the winter period.

Using a quick calculation, over two months, a pasture growing at 50 kg DM/ha/day will grow 3000 kg DM/ha. At 0.3% S (i.e. maintaining S-efficiency) this means the pasture requires 9 kg S/ha over this two month period. Applied at 100 kg/ha, n-rich ammo 40N would supply approximately two-thirds of the sulphur requirement over this period, meaning the soil would only have to supply around 3 kg S/ha and therefore the pasture response to the applied N overall would unlikely be limited by S in this situation.