Guide to slurry assessment and application

Livestock slurry is an important source of nutrients for grassland and arable crops.

Below we investigate how farmers can make the best economic and environmental gain from this key resource by taking a look at:

How to assess nutrient content

Assessing the nutrient content of slurry is a key starting point, because this will determine how much you spread.

The Nutrient Management Guide (RB209) lists the total and available figures for nitrogen, phosphate and potash (N, P2O5 and K2O) at different dry matter (DM) content.

Elaine Jewkes, British Grassland Society director says it is vital to consider the DM percentage.

“If a gallon of slurry is 98% water it will have a lot less nutrient content than something that is 90% water and 10% DM,” she says.

Using “book” figures, such as those in RB209, should be the minimum approach to working out actual nutrient content of applied manures; Ms Jewkes advises using freely available tools such as MANNER-NPK to make this easier and quicker.

“Use book values or a slurry meter and hydrometer for a basic plan then refine afterwards,” Ms Jewkes advises.

Soil test results should be used as well to correctly account for the nutrients – those required and applied.

Options for assessing precise analysis of slurry

  • Simple on-farm analysis can be carried out using portable meters to assess readily available N, and slurry hydrometers for calculating DM and, from this figure, P2O5 and K2O content.
  • For the “gold standard”, obtain a laboratory analysis of the manure or slurry
  • For all but the most dilute slurries, rapid near-infrared spectroscopy analysis, a method also used for analysing fresh grass samples, can provide a quicker result
  • Standard “wet chemistry” can be used but this will take a few days to come through

How to sample slurry

Safety must be a priority – for instance ensure there is a platform to stand on when taking a sample from a lagoon; if there is somewhere safe to stand a weighted vessel can be used, but safety must be paramount, says Ms Jewkes.

“Definitely don’t enter an enclosed slurry space – toxic gases can build up in slurry stores and they will quickly render you unconscious.”

Slurry becomes stratified during storage so mixing it will give the best sample.

Alternatively, take several samples. One way could be from the pipe after pumping to the tanker – and mix in a bucket; this will give an overview of the slurry that has been applied.

Tips for sampling

  • Even if you can mix the store you should take more than one sample, bulk it, then sub-sample
  • Between sampling, cover the container of mixed samples with a lid to reduce nitrogen losses
  • Never completely fill the sample container for the lab as gases will form and this could result in the lid popping off in transit

How to sample farmyard manure

Farmyard manure (FYM) is normally safer to sample, although climbing on heaps is not advisable as they can be less stable than they appear.

Take several samples, but not directly from the surface as this will be oxidised.

Mix the samples and subsample – the new RB209 provides good advice on this.

How much slurry should I apply?

Best practice dictates a maximum of 50cu m/ha (4,500 gallons/acre) although this is at the upper limit.

Ideally plan what you need and how many times you want to apply, says Ms Jewkes.

“If you are cutting grass for silage for instance, consider what nutrients are needed and plan a sensible dose of nutrients taking the soil test result into account.

“You can often supply all of the P and much of the K needed for a decent first cut, meaning that only NKS [nitrogen, potash, sulphur-containing] fertiliser is required.

“This is an example of where planning how nutrients in slurry are used is a very cost-effective approach.”

Once the closed period for slurry application ends in nitrate vulnerable zones and until the end of February, no more than 30cu m/ha (2,700 galls/acre) can be applied in a single dose, with at least three weeks between applications.

“In the early and later part of the year the nutrients won’t be as well utilised so keep application rates as low as possible,” Ms Jewkes advises.

“In practice I’d rarely recommend much more than 30-35cu m/ha for any application, and less if used on grazing during the season.”

When should I apply it?

Timing is everything, Ms Jewkes insists.

“Check the weather forecast as applying before rain doesn’t wash it in, it tends to wash it off, and with this comes the potential for watercourse pollution.”

How do I calculate what quantity I have applied?

The best tool for this is a flow meter on an umbilical system – this gives a straightforward application per field.

This can be used to calibrate spreading rates too. Cover a known area and check the rate, to ensure you are spreading at the rate you planned to.

If you haven’t got a flow meter, log tanker loads per field and divide the total applied by the area to get the rate.

“The same goes for FYM. You can calibrate the spreader using trays of a known size laid out along a measured distance of a bout to find the spreading rate.”

When slurry is applied, how long should the grass be left before it is grazed or cut for silage?

For grazing, Ms Jewkes advises at least three to four weeks – if a low-emission spreader such as a trailing shoe has been used it can be at the lower end of this, but don’t graze if contamination is visible as it will increase rejection and may cause problems due to pathogen transfer to the animal.

For silage, the longer the time lapse between application and cutting will generally mean that surface contamination is gone, unless the weather has been exceptionally dry.

But leaving sufficient time for N to be fully used by the grass is important.

Spreading methods compared

  Pros Cons Situation
  • Easy to use
  • Range of DMs can be spread
  • No hoses to clog
  • High ammonia losses therefore lower nitrogen use efficiency
  • Sward contamination
  • High potential for run-off if it rains after spreading
  • Strong odour
  • Possibly harder to spread accurately
  • Suitable for all land uses
Trailing shoe
  • Easy to use
  • Can be used repeatedly because no soil slitting
  •  Low sward contamination
  • Reduced ammonia losses therefore more available nitrogen
  • Low odour
  • Bout width known so spreading accuracy easier
  • Relatively inexpensive to buy as a basic unit
  • Thicker material can be tricky so may need dilution
  • Pipes can clog with thicker material
  • Only really suitable for grass as the shoe would damage a growing crop because this method is designed to comb through the sward
Trailing hose
  • Similar to trailing shoe
  • Similar to trailing shoe
  • As the hoses are above ground level it can be used in the early stages of a growing crop so might be better suited to a mixed-farm situation
Shallow injection
  • Known bout width
  • The least sward contamination and lowest ammonia losses of all methods
  • Minimal run-off potential although slits may provide a channel in very poor conditions
  • More expensive to buy and also to use because it takes a lot of tractor pulling power and therefore consumes more diesel
  • Timing of application must be accurate to avoid situations such as slits not closing up when conditions are dry after application




  • May be difficult on some soils, in particular stony conditions, because of wear or damage to cutting discs
  • In heavier soils, there is the risk of smearing slits and consequential damage to soil