Managing Livestock Manures 4: Managing Manure on Organic Farms

Nutrient losses - a waste of a valuable resource and environmentally damaging

Animal manures (including slurries) are valuable sources of nutrients in organic farming systems. Careful management is required to reduce the potential risk of pollution to the environment:

Point source pollution of water courses can occur as a result of a burst or overflowing slurry store and yard or field runoff during heavy rainfall events. Such incidents can have catastrophic effects on fish and other aquatic life, because of the direct effect of dissolved ammonia (from the manure), or by depletion of oxygen levels in the water as the added organic matter is broken down.

Diffuse pollution is the pollution that can arise when nutrients are lost from fields, is not easily seen and can affect water and air over a large area.

Nutrients of concern

Nitrate (NO3) affects water quality. It can contribute to eutrophication of surface waters (e.g. development of algal blooms) and can exceed legal limits in drinking water. Losses from agriculture are a major source of nitrate in some ground and surface waters, especially in eastern and central England.

Ammonia (NH3) loss as gas (volatilisation) can cause a number of environmental problems. NH3 can be redeposited either locally or transported long distances. This aerial deposition can disturb the balance in natural ecosystems by adding nitrogen. It also causes the acidification of soils and water bodies and so contributes to acid rain. Livestock production is the most important source of ammonia emission in Northern Europe.

Nitrous oxide (N2O) is a greenhouse gas, i.e. contributing to global warming. It is also implicated in the destruction of ozone through reactions with chlorinated carbon compounds.

Phosphorus enrichment is the main factor contributing to algal blooms in freshwater systems. These can be damaging to aquatic life, unsightly and in some cases poisonous to humans and animals. Agriculture contributes significant phosphorus loads to surface waters, due to run-off and leaching.

Potassium is an important plant nutrient; small losses do occur by leaching but have not been associated with a risk to the environment.

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Losses occur through all stages of manure management (Figure 5).

Ammonia is lost to the atmosphere at all stages. Release to the atmosphere can be rapid. Nitrate is produced in soil after land application. It is highly mobile and can be washed out with drainage water. Losses of dissolved N (and other nutrients) from housing, manure storage or handling only occur if run-off is not collected. Nitrous oxide is derived from the microbial breakdown of nitrate and this emission is encouraged by the presence of manure. Phosphorus is normally held firmly in soil, but excessive applications can result in P leaching. Losses also occur by soil erosion and from surface run-off of freshly applied manure. These are the major routes for phosphorus loss.

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