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Nutrient management: Getting nutrients exactly right on grassland

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Ensuring grassland is provided with exactly the right nutrients not only makes sense financially, but will also bring big benefits to the bottom line.

Understanding what nutrients you have on-farm in the form of farmyard manure is key.
Understanding what nutrients you have on-farm in the form of farmyard manure is key.

Drawing together a nutrient management plan based on up-to-date soil analysis results not only makes financial sense, but is also now a mandatory requirement for all farmers.


Under ‘The Farming Rules for Water’, which came into effect in April last year, all ‘cultivated land’ must be soil sampled a minimum of every five years (see soil analysis panel).


This includes any land – such as permanent pasture – which has had fertiliser or manure applied within the last three years. These soil tests must be used as the basis for a nutrient management plan.


Philip Cosgrave, grassland agronomist for Yara, says there are some strong reasons for adopting such a approach.


“We need to start growing grass with the same detail as we grow a crop of wheat. This means we need a plan in place to optimise each cut of silage,” he says.


Agri-Food and Biosciences Institute soil scientist Suzanne Higgins puts the cost of a standard soil test at about £5/hectare (£2/acre) when labour is costed in.


If this can be used to save purchased fertiliser costs, which can be more than £200/ha (>£80/ acre), depending on the formulation, then the potential return on investment are marked.


Only by soil testing will producers truly understand the individual nutrient requirements of each field, and that is where the ultimate savings will be made.


Monitoring soil indexes from year to year will highlight where soil fertility is declining, and this is due to crop off take over time being greater than what has been applied. Knowing this you can then redistribute nutrients within your farm to greater effect.


“We’re trying to achieve or maintain a P index of 2 and a K index of 2-. Whether it’s a threeor five-cut system, the aim is to maximise the yield and quality from each forage hectare.


“During peak grass growth [>100kg/DM per day] in April and May the grass crop has a daily phosphate and potassium requirement of 0.7 and 3.6kg/ha respectively.


If the soil can’t release these levels of P and K daily then nitrogen cannot be used efficiently to drive high levels of daily DM production. Yield reductions of up to 30% can result if the crops P and K requirements are not met,” he says.

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Soil analysis – what and when

  • The Farming Rules for Water state that soils must be tested for phosphate, potassium, magnesium and nitrogen every five years.
  • However, SNS calculations from the RB209 Fertiliser Manual can be used to establish N levels. Taking a soil sample correctly is crucial because it will impact results.

How to take samples:

  • Sample between October and November
  • Do not sample within three months of applying organic manures or mineral P and K fertiliser
  • If lime is applied, it is best to wait 12 months for a stable soil pH reading
  • Sample at least every five years, and preferably every three years
  • Take one sample per four hectares (9.8 acres). Divide fields greater than 4ha (9.8 acres)
  • Take separate samples from areas that are different in soil type and cropping history
  • Use a soil corer to sample to a depth of 7.5cm on long-term grassland fields
  • Sample the area, walking in a W pattern
  • Avoid headlands, gate entrances, or where manure or lime was heaped
  • Take 20-25 soil cores from the sampling area
  • Place soil cores in the sample bag and label clearly with the farm and field name

Nitrogen recommendations for grass silage
Nitrogen recommendations for grass silage

Soil testing is the first part of a four-part process for formulating a farm nutrient management plan. The RB209 Fertiliser Manuals form the basis for all calculations and should be referred to for the subsequent steps:


Calculating crop requirements

Use RB209 Section 3 Grass and forage crops to establish the nutrient requirements of specific crops.


Phosphate (P) and potash (K)

There are two tables in RB209 (page 10) which provide guidance on silage ground P and K requirements depending on whether you know your expected grass silage yields or not. If you are unsure, use the standard values for P and K recommendations after each silage cut. Grazing ground figures are available separately.


Nitrogen (N)

Use the Soil Nitrogen Supply (SNS) calculations in the RB209 Nutrient Management Guide to calculate nitrogen levels.

  • The SNS will depend on previous management and nitrogen use. On dairy farms which are likely to be using >250kg N/ha/ year, it is likely the SNS will be ‘high’ or ‘moderate’.
  • Grass growth class (GGC) of the ground should then be established – this describes the ability of the site to respond to nitrogen, which depends on soil type and rainfall. For example, good or very good GGC sites are swards dominated by productive grass species which respond well to increased nitrogen supply as soil drainage, temperature and water supply are conducive to growth.
  • Table 3.8 can then be used to establish nitrogen application rates, depending on target dry matter yields. These recommendations are applicable to grass swards with low clover content in a very good/ good GGC and moderate SNS situation.
  • Poor GGC sites are likely to achieve DM yields towards the lower end of the range in most years.
  • Application rates should be adjusted depending on SNS sites. For example, on low SNS sites, 10kg N/ha more should be applied for first cut and 20kg N/ ha more for second cut.



Sulphur is considered an essential nutrient in maximising dry matter yield and protein percentage in both grazed and conserved grass (see Use sulphur as standard panel).


Sulphur should be applied on all grass grown on:

  • Sandy and shallow soils.
  • Loamy and course silty soils in areas with >200mm rainfall between November and February. JClay, fine silty or peat soils in areas with >400mm rainfall between November and February. On soils at risk of sulphur deficiency apply:
  • Silage: 40kg SO3/ha before each cut.
  • Grazing: 20-30kg SO3/ha when up to 100kg N/ha is applied and an additional 20- 30kg SO3/ha for each additional 100kg N/ha applied.
Table 1: The value of different dry matter slurries Using
Table 1: The value of different dry matter slurries Using

Knowing the value of farm organic matter

The next step is understanding what nutrients you have on-farm in the form of slurry and farmyard manure (FYM).


The RB209 Fertiliser Manual Section 2 Organic Materials, provides guide values for the nitrogen levels of a typical dry matter slurry. However, Mr Cosgrave says it is well worth testing your farm’s supplies as in practice, NPK levels, can vary considerably.


He explains: “The standard book value for cattle slurry is 6% dry matter, but on two grass trials last year in Derbyshire and Devon, where we tested each farm’s slurry, one had a dry matter of 3.3%, while the other was 1.56%. That shows how you can be way off the mark by assuming a standard book value.” (see Table 1).


He advises testing slurry and FYM at least two to three times a year and using this to balance mineral fertilisers.


Knowing how much mineral fertiliser needs to be applied

Knowing how much mineral fertiliser needs to be applied


You can subtract the nutrients that were applied in the manure from the crop requirement to work out the shortfall needs to be filled with mineral fertiliser (see Table 2).


Mr Cosgrave urges producers to select a mineral fertiliser based on what their soils require, rather than reaching for the same product year-on-year because it is what they have always used.


“Split the farm up into areas with different P and K indexes and formulate mineral fertiliser recommendations based on that,” he advises.


Possible scenarios, where a traditional fertiliser grade would not be the right choice might be on a silage field with a P index of 0 or 1. In this case, a high P compound like a 20-8-11+S should be used instead of a traditional silage grade to build soil P reserves.


A field with a high P index (3 or more) might not need mineral P if slurry was applied, so a zero P silage grade would be best suited for this example.

Use sulphur as a standard

Including sulphur in mineral fertilisers as standard will help boost nitrogen use efficiency, dry matter yields and grass protein levels, according to Yara’s Philip Cosgrave.


Since the reduction in atmospheric sulphur dioxide emissions, the level of sulphur in the air – and hence the soil – has declined substantially. This has resulted in more than 80% of UK grassland soils being deficient in sulphur.


The two essential plant amino acids, cysteine and methionine have a requirement for Sulphur, so if levels are lacking, plant proteins are not formed optimally.

  • Methionine initiates the synthesis of proteins
  • Cysteine is critical for structure, stability and catalytic function in proteins

Mr Cosgrave says: “When you apply sulphur on grassland, you typically get an increase in dry matter yields, but also an increase in protein percentage.


If grass has an adequate supply of sulphur, more of the nitrogen is found as protein. Whereas, sulphur deficiency results in less of the nitrogen being found as protein and more in the form of nitrate and free amino acids.”


Soil type will effect its responsiveness to sulphur, however in general, farmers could expect to see a 10-30% increase in dry matter yields from sulphur.


First cut silage has a particularly high demand for sulphur and regardless of soil type should receive a sulphur application.


Medium and light soils, and soils with lower organic matter content show the greatest response to sulphur in subsequent cuts.

Boosting fertiliser efficiencies – key considerations

Boosting fertiliser efficiencies – key considerations

Soil pH

Maintaining the optimum soil pH has many benefits, including:

  • Increased biological activity of soils which increases the turnover of major nutrients. Grassland soils can release up to 80kg/ha more nitrogen
  • Makes P and K more available in the soil
  • Increases earthworm activity, which improves soil structure
  • Increased survival and productivity of rye-grass

Improving N use efficiency

  • The use of low emission slurry spreading equipment reduces gaseous nitrogen (N) losses by up to 50%. This can reduce your mineral N requirement
  • The type of mineral N used plays an important role in determining nitrogen use efficiency. Urea is prone to ammonia losses which is significant particularly if it does not receive 5mm of rainfall in the 24 hours following application
  • Nitrate-based products have very low ammonia losses and thus more N is available for the crop

In numbers

  • K and S deficiency can result in yield losses of 30%. So if your total annual yield was 10t DM/ha this might reduce to 7t DM/ha/year due to K or S deficiency. You could be losing about £150-£270/ha (about £50- £90/t of good quality silage)
  • A sulphur-containing fertiliser might cost an extra £5/ha per cut, but could prevent yield losses worth up to £90/ha per cut jSoils at sub-optimal pH could be losing 2t DM/ha/year. The extra grass potentially achievable through correcting soil pH is potentially worth a five-fold return on the lime investment

(Source: AFBI)

Gibberellins boost early growth

Applying gibberellins to grassland in early spring could boost grass growth by 20kg DM/ha/ day or more and help those farmers wanting to quickly fill a forage gap this spring.


Gibberellins are naturally occurring growth enhancers that all plants produce. They help to increase cell size and numbers, enabling greater photosynthesis, plant metabolism and, in turn, increase growth in leaves and roots.



NuFarm’s Brent Gibbons adds: “If grass has been heavily grazed, the plant produces gibberellins to try and boost grass growth.”


UK research on 10 farms showed that a spring application resulted in an average extra 500kg DM/ha over a 21-day period. This led to a 8:1 return on investment thanks to grass growth and milk yield.

  • Gibberellins such as SmartGrass, should be sprayed before the season warms up when conditions are at 5-10degC and grass growth is limited
  • Applications must be made before April
  • Gibberellins have a three week impact on the sward – grass should be utilised within this period

Mr Gibbons believes gibberellins have a role to play on most farms.


He says: “It’s relevant to all grassland farmers and especially ones short of forage that want to get off to an early start this spring.”

Essential resources

For more details on nutrient management planning, view the RB209 Fertiliser Manuals at

You can also view practical videos on how to take a slurry and FYM samples.


Read the full Farming Rules for Water rules at

Grassland Toolkit 2019

Grassland Toolkit 2019

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