Optimising The Fertiliser Value Of Slurry

17th October 2021

In these times of high fertiliser prices and in the light of the environmental impact of poor slurry management on water quality and ammonia emissions, it is very important that farmers realise the economic value of slurry and know how to maximise its value as a source of nutrients, for growing crops.

The nutrient value of slurry is measured by the dry matter content of slurry (the higher the dry matter the more nutrients in it), and by the N, P and K content. While the nutrient content of typical cattle slurry is in the following table, it is important to realise that, in practice, these values can vary enormously owing to variations in animal type, diet and slurry dilution with water. For these reasons, it is essential that a slurry sample (from an agitated tank) is sent to the laboratory for analysis.

Typical dry matter and nutrient content of cattle slurry in Ireland (Teagasc 2020)

Mean content

Typical availability

Available Kg/m3

Available units/ 1000gals

Dry Matter






6 units



50% (Index 1 & 2)

100% (Index 3 & 4)


0.5 kg

2 units

5 units





32 units

Reducing Ammonia Emissions in the Tank

40-60% of the nitrogen in slurry is in the form of ammonia. Ammonia is continually being emitted or volatilised from slurry tanks, especially if they are uncovered. This represents a significant loss of nitrogen. Ammonia gas, when it gets into the atmosphere, falls back to ground level and can cause eutrophication of sensitive habitats and watercourses. By adding a product called Digest-It to the slurry, we can significantly reduce the ammonia emissions from the slurry tank. Digest-It is an-inoculant which utilises ammonium in the slurry for its own propagation, in effect converting ammoniacal nitrogen into microbial nitrogen. Unlike ammonia, microbial or organic nitrogen is a more stable form of nitrogen that is released slowly to grass over a longer time period. Recent trials conducted by Devenish have shown that the addition of Digest-It to slurry has reduced ammonia emissions by 80%. As the smell of slurry is associated with the ammonia levels in the slurry, the use of Digest-It will reduce the offensive smell of slurry significantly.

Reducing Ammonia Emissions in the Field

In order to further reduce losses of nitrogen in the form of ammonia we should realise that weather conditions at time of spreading are critical and we should aim to spread slurry when conditions are cool, overcast and even misty. Avoid spreading on hot, sunny days, where possible. Apply at a time when crop demand is high. Aim to have 70% of slurry applied by the end of April.

Spreading slurry using "low emission application methods" such as trailing shoe, band spreader or shallow injection methods will further reduce ammonia losses by between 25-35% compared to Splash plate spreading, which is going to be banned in 2023.

The final point in maximising the value of slurry is to target areas for slurry spreading, based on soil analysis.

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