ARCHIVE: Fertiliser Manual (RB209)

Protection of the environment

 


 

Farmers are encouraged to adopt a systematic approach to fertiliser planning to help minimise losses of nutrients from agricultural production systems which can, directly or indirectly, have adverse effects on water and air quality with wide ranging effects on loss of biodiversity. Recommendations in this Manual for the use of fertilisers and manures are based on economic criteria but matching nutrient applications and soil levels as closely as possible to crop requirement will go a long way to help minimise the risk of nutrient loss from soil to water or to air. Practices that make the best economic use of nutrients also help protect the wider environment. These include:

  • Regular soil analysis every 3-5 years for P, K and Mg Index and pH.
  • Identification of the SNS Index every spring before applying nitrogen fertiliser.
  • Estimation or measurement of the nutrient contents of any organic manure applied.
  • Taking account of all other sources of nutrients before deciding on fertiliser application rates.
  • Where appropriate, soil or plant tissue analysis to help with decisions on application of sulphur or micronutrients.
  • Use of a recognised nutrient recommendation system.
  • Regular calibration and tray testing of fertiliser spreaders and calibration of manure spreaders.
  • Rapid incorporation of organic manures after application to tillage land or use of trailing hose, trailing shoe or injection equipment for slurry.

Some areas of agricultural land are subject to various types of regulation or agreement for maintenance or improvement of the environment and farmers may have to comply with restrictions on the use of liming materials, fertilisers and manures. Farmers in these areas may have to modify the recommendations in this Manual to comply with any specific requirements.

Nitrate

The EC Nitrate Directive, adopted in 1991, requires Member States to introduce controls on agriculture in water catchments where the nitrate (NO3) concentration in waters either exceeds, or is at risk of exceeding, 50 mg/litre or where there is a risk of eutrophication of surface waters. To comply with these requirements, Nitrate Vulnerable Zones (NVZs) have been designated in the UK. Since December 1998, farmers of land within these areas have had to comply with mandatory NVZ rules (Defra NVZ Guidance Booklets – see Section 9).

Within NVZs, there are closed periods during which manures containing more than 30% readily available nitrogen (cattle or pig slurry, poultry manures and liquid digested sludge) must not be applied although limited applications may be made by registered organic farmers. Closed periods also apply to manufactured fertiliser nitrogen though there are some specified exceptions.

Also within NVZs, the average crop-available nitrogen application rate in manufactured fertiliser and manure to certain crop types must not exceed specified (Nmax) limits.

An important requirement of the Action Programme is that the amount of crop-available nitrogen applied in manufactured fertiliser and manure should not exceed crop requirement and that full allowance should be made for nitrogen available from the soil organic matter and previous crop residues.

Incidental losses of nitrogen to waters should be minimised. This type of loss can be related to a specific farm activity such as manure or fertiliser application that has taken place under unsuitable conditions. Fertilisers or manures should not be applied to steeply sloping, frozen hard, snow-covered or water-logged soil or during rain as these conditions greatly increase the risk of run-off.

The risk of loss of nitrate by leaching can be reduced by ensuring that the amounts of nitrogen applied from all sources are no greater than the crop requires and by applying nitrogen in organic manures and fertilisers under suitable conditions close to the time when the nitrogen in them is needed for crop growth.

Getting nitrogen fertiliser applications right is important for reducing the amount of nitrate leached but choice of crop, autumn crop cover, cultivations, organic manure use and grassland management all have a major impact on the quantity of nitrate lost. Accurate records of past fertiliser use and the regular calibration of fertiliser application machinery will increase the accuracy of fertiliser decisions and application to land.

Ammonia

Agriculture is by far the largest source of all ammonia emissions to the atmosphere, mainly from livestock manures but also from nitrogen fertilisers, especially urea. Ammonia is emitted from solid manures and slurries in livestock housing, stores and following manure spreading to land. The greatest losses are from livestock housing and spreading manures.

Following emission, ammonia can be deposited onto land or water, either nearby or after being transported in the atmosphere over considerable distances. Emission of ammonia can be damaging to the environment because it:

  • Contributes to acidification of the soil.
  • Adds nitrogen to habitats (e.g. heathland) that are damaged by nutrient inputs.
  • Is a precursor to fine particles of ammonium sulphate which have impacts on human health.

There are strong pressures within the EU to reduce ammonia emissions from agriculture. Large pig and poultry farms are already covered by permits (Environmental Permitting (England and Wales) Regulations 2007, SI 3538), which implement the EU Integrated Pollution Prevention and Control (IPPC) Directive. There may be controls on a wider range of farms in the future.

The most effective means of reducing ammonia emissions are to:

  • Incorporate solid manure and slurry into the soil on tillage land, preferably by ploughing soon after broadcast spreading.
  • Apply slurries with an injector or band-spreader (trailing hose or trailing shoe) (see Section 2).

These ‘low emission’ spreading techniques reduce ammonia emissions typically by 30-70% compared to conventional ‘broadcast’ spreading. They are more cost-effective than modifying housing or stores. Where control measures are implemented for housing or stores (e.g. fitting a store cover or roof), it is especially important to use low emission spreading techniques to prevent the conserved ammonia from being lost when the manure is applied to land.

Where possible, urea should not be applied under conditions that promote ammonia loss: warm, drying soils and calcareous soils.

Adopting these techniques to conserve ammonia from late winter and spring applications of manures and slurries will also improve the supply of available nitrogen for crop uptake and will decrease the need for nitrogen fertilisers.

Nitrous oxide

Nitrous oxide (N2O) is a potent greenhouse gas that is emitted naturally by all soils. Greenhouse gases are associated with climate change and there is strong pressure to limit their emission. The concentration of nitrous oxide in the atmosphere has increased over the past century and part of this increase is attributed to agriculture. Nitrous oxide is formed in soil during nitrification (conversion of ammonium-N to nitrate-N) and denitrification (conversion of nitrate to nitrogen) and its formation tends to be greatest where soil aeration allows both of these processes to occur. This tends to be at water contents around field capacity. Emission of nitrous oxide from the soil tends to increase following the addition of crop residues, manures, fertilisers or any form of mineralisable organic matter that will produce ammonium-N or nitrate-N.

Emission of nitrous oxide can be minimised by ensuring ammonium-N and nitrate-N concentrations in the soil do not exceed crop requirement. Using the amount of nitrogen recommended in this Manual, together with care in fertiliser and manure application, will help minimise both nitrous oxide emission and nitrate leaching.

Phosphorus

There is increasing environmental concern over the phosphorus status of many inland surface waters and the role of agriculture as a source of phosphorus. Phosphorus is an important nutrient with regard to water quality because small increases in concentration can cause eutrophication (nutrient enrichment) of fresh waters. The effects of eutrophication include algal blooms, fish death, excessive weed growth, poor water clarity and loss of species diversity. These effects are visually unattractive, interfere with water use and ecology, and can be hazardous to animal and human health. The EU Water Framework Directive is focussing attention on the need to control eutrophication due to phosphorus movement from soils by requiring all surface waters to have good ecological and chemical status by 2015.

Wastewater from sewage treatment works and agriculture are the main sources of phosphorus in surface waters. Research has shown that the amount of phosphorus transferred from agricultural land can be sufficient to cause eutrophication. Phosphorus moves from soil to water by:

  • Surface run-off of recently spread fertilisers and manures
  • Erosion of soil particles containing phosphorus
  • Particulate and soluble phosphorus in drain outflows K

Key measures to reduce the risk of phosphorus movement to water are:

  • Following the recommendations in this Manual to maintain the target level of crop-available soil phosphate and avoiding any unnecessary build-up above the target Index (see page 37) and taking full account of the phosphate content of organic manures (see Section 2).
  • Minimising the risk of soil erosion by following advice contained in The Code of Good Agricultural Practice and Controlling Soil Erosion (MAFF PB 4093) and guidance in 'thinksoils' from the Environment Agency.
  • Avoiding surface applications of all organic manures (solid or liquid) when soils are snow-covered, frozen hard, waterlogged, deeply cracked, or on steeply sloping ground adjacent to watercourses (see The Code of Good Agricultural Practice).
  • Applying inorganic fertiliser in appropriate amounts as annual dressings rather than as a single, large dressing, except where the aim is to increase the soil Index. Such applications should be ploughed in. If there is a significant risk of surface run-off entering watercourses (e.g. poorly drained clay soils on sloping land) phosphate fertilisers should not be applied when there is a risk of heavy rainfall. This will reduce the risk of phosphorus run-off which can pollute surface waters.

 

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