4: Slurry as a pollution problem

At the end of this session you will be able to:

  • Describe how slurry affects the environment.
  • Explain the impact of slurry pollution on the environment.
  • Define the term BOD-Biological Oxygen Demand.
  • Describe how the slurry affects the soil biota (small microorganisms).

Why is slurry a pollutant?

A mixture consisting of dung and urine, livestock bedding, rainwater and washings from a building or yard used by livestock is called slurry.

It can vary in Dry Matter content and nutrient content.

It contains microbes and has a high BOD – Biological Oxygen Demand. This means that when it is in a river or stream it takes away the oxygen, and thereby suffocating the fish.

Biological Oxygen Demand


Biochemical oxygen demand (BOD), the amount of dissolved oxygen used by microorganisms in the biological process of metabolizing organic matter in water. The more organic matter there is (e.g., in sewage and polluted bodies of water), the greater the BOD; and the greater the BOD, the lower the amount of dissolved oxygen available for higher animals such as fish.

They then ‘suffocate’ in the water through lack of oxygen.

As a pollutant in the rivers.


“From 1 January, there is a new requirement to not spread slurry when heavy rain is forecast within 24 hours.”

In Wales, farmers are being encouraged to stave off the threat of expansion of nitrate vulnerable zones (NVZs) by following guidance on slurry spreading after prolonged wet weather and temporary slurry storage.

© Ffermio / Telesgop / S4C

© Ffermio / Telesgop / S4C

As an air pollutant


Emissions of ammonia have been on the rise in the UK, new statistics from the government show, even while the amount of other pollutants entering the atmosphere has fallen.

Most emissions of ammonia come from agriculture, in the form of a gas produced by slurry or other rotting farm waste and fertiliser. It is a cross-border pollutant as it can be carried on the wind.

In the air it is regarded as a powerful pollutant because of its capacity to combine with other pollutants in the atmosphere to create very small particles, known as PM 2.5, which can be highly harmful to the lungs when inhaled – particularly by people who are already vulnerable.

As a pollutant in soil

Too much slurry can kill all the organisms in the soil.

Soil organisms

Micro-biota - Soil algae, bacteria, fungi, protozoans

Mesa-biota - Nematodes, oligochaetes, insect larvae, collembola

Macro-biota - Earthworms, rats, snakes

pH Levels in the Soil

Slurry has a very acidic pH.

Too much slurry makes the soils acidic which is not ideal for grass and clover growth.

It also affects the numbers of earthworms in the soil.


  • Structure the soil.
  • Increase fertility.
  • Decrease erosion.
  • Deepen the roots.

Decaying of microorganisms

  • Makes the soil thick and rich.


Pollution is the damaging consequence of materials affecting the air, soil, and water layers of the earth.

Farmers can affect the levels of pollution in the environment.

Slurry, a by-product of livestock farming, has very harmful side effects when it pollutes.

Due to its seriousness as a pollutant great care must be taken with its storage and utilization on the land.