Ammonium nitrate fertiliser
Knowledge and understanding
|Ammonium nitrate fertiliser||
Understand all associated hazard knowledge
Pure ammonium nitrate (NH4NO3) is a white, odourless salt with a melting point of about 170°C. In its solid form it is stable to mechanical shock and does not burn. Ammonium nitrate is a potentially explosive substance because it is a combination of the oxidising nitrate ion, in intimate contact with a fuel, the ammonium ion. Further information is provided in the Foundation for hazardous materials – Ammonium nitrate based fertiliser.
Fertilisers with a high nitrogen content can be misused as improvised explosives and pose safety risks if mishandled in manufacture, transport or storage.
The hazards of ammonium nitrate include:
- Dangerous reaction with many materials, potentially causing fires and explosion
- Toxic gases, with high-temperature decomposition producing:
- Nitric acid vapours
- Nitrogen dioxide
- Environmental pollution
- Health hazards:
- Moderately toxic if ingested
- Continued direct contact can result in skin irritation
If heated by an external fire, the explosion sensitivity of the decomposing ammonium nitrate increases dangerously as the temperature increases. This hazard is amplified if:
- The ammonium nitrate contains other ingredients or contaminants
- The ammonium nitrate mixes with contaminants or fuel; this may happen more readily if the ammonium nitrate has become molten
- Molten ammonium nitrate is trapped, for example, in drains or machinery, which means that decomposition gases cannot escape
The risk of fire leading to an explosion is greatly increased if ammonium nitrate is mixed with combustible or incompatible materials, such as:
- Powdered metals
- Alkali metals
- Chromium or copper salts
- Organic and carbonaceous materials
- Reducing agents
Fires in ammonium nitrate do not necessarily result in an explosion. Fires involving ammonium nitrate have caused accidental explosions, usually as the result of an uncontrolled fire due to the presence of combustible substances. Accidental explosions without an uncontrolled fire, as a result of reactions between ammonium nitrate and incompatible materials, are the exception.
Packaging and labelling
Fertiliser packaging is required by law to include a number of details, including:
- The prescribed name of the product content, such as ‘NPK Fertiliser'
- The major nutrient contents, nitrogen (N), phosphorus (P) and potassium (K)
- Forms in which the nitrogen content is present, such as ammonium nitrate
- Solubility of the phosphorus content; P and K are usually expressed as their oxide content, followed in brackets by the content of the element
- Levels of secondary nutrients present, such as magnesium (Mg) and other trace elements
- Any pesticide content, as well as a statutory declaration to comply with pesticide legislation
- The name and address of the manufacturer
- The guaranteed weight of the product
- An EC product declaration, if the product is EC approved
A code will be found on the bag, for example, 26-23-29. The numbers, in order, refer to:
- N% of nitrogen
- P% of phosphorus as if it were phosphorus pentoxide (P2O5)
- K% potassium as if it were potassium oxide (K2O)
The first number, nitrogen, is the one of main interest to fire and rescue services. The higher it is, the greater the volume of nitrates or ammonium compounds, and thereby the greater the danger from the mixed fertiliser if it is near to or involved in a fire.
Historically, any nitrogen figure of 20% or more, up to a maximum of 35%, has been regarded as a potential explosive risk if the fertiliser is near to or directly involved in a fire. However, it is possible for fertilisers with less than 20% nitrogen to explode if they have been contaminated or pressurised before being heated.
Some ammonium nitrate fertiliser carries an oxidising warning symbol. If this symbol is displayed, the fertiliser will be classed as a dangerous substance.
Agricultural fertiliser groups
Fertilisers that contain 28% or less nitrogen, as detailed on the label or safety data sheets (SDS), do not normally present an explosion hazard. Therefore, in an agricultural context, ammonium nitrate-based fertilisers can be divided into two groups to identify the precautions required:
- Fertilisers that contain more than 28% nitrogen – most of these are straight ammonium nitrate types, although they include a small number of compound fertilisers
- Fertilisers that contain 28% or less nitrogen – compound fertilisers form the major proportion of this group; the straight nitrogen types are usually a mixture of ammonium nitrate with limestone or similar inert materials
Ammonium nitrate fertilisers may be transported by road, rail or sea. For more information, including details of the signage that should be displayed, refer to the Foundation for hazardous materials - Explosive hazards.
Legislation for Great Britain
Technical grade ammonium nitrate, is classified as a 'Class 1 explosive' in the UN European Agreement concerning the International Carriage of Dangerous Goods by Road.
The Ammonium Nitrate Materials (High Nitrogen Content) Safety Regulations prohibit the importation and supply of relevant ammonium nitrate with more than 28% nitrogen without a supporting detonation resistance certificate.
The Fertilisers and Ammonium Nitrate Material (Amendment) (EU Exit) Regulations amends legislation relating to fertilisers, addressing failures of retained EU law to operate effectively and other deficiencies arising from the withdrawal of the United Kingdom from the European Union.
The handling and storage of ammonium nitrate fertiliser is covered by The Notification of Installations Handling Hazardous Substances Regulations. The storage of ammonium nitrate fertiliser that is classified as a dangerous substance, with the oxidising warning symbol, is covered by The Dangerous Substances (Notification and Marking of Sites) Regulations.
In some situations, the fire and rescue service or enforcing authority (the Health and Safety Executive or local authority) will need to be notified about the storage of ammonium nitrate fertiliser:
- If there is a nitrogen content of up to 15.75%, there is no requirement to notify either the fire and rescue service or the enforcing authority
- If the nitrogen content is over 15.75% but it is not classified as a dangerous substance:
- The fire and rescue service needs to be notified if there is more than 150 tonnes
- The enforcing authority does not need to be notified
- There does not have to be a warning sign at the site entrance
- If the nitrogen content is over 15.75% and it is classified as a dangerous substance, with an oxidising warning symbol:
- The fire and rescue service and the enforcing authority need to be notified if there is more than 25 tonnes
- There has to be a warning sign at the site entrance
Further information and examples about the storage of ammonium nitrate fertiliser is provided on the Health and Safety Executive website - Notification of dangerous substances and ammonium nitrate on farms.
Legislation for Northern Ireland
GOV.UK provides guidance on the Marketing and use of explosives precursors in Northern Ireland. Legislation is provided under .
Tier 1 substances require a licence for supply, acquisition, possession, use or import into Northern Ireland. The tier 1 substances include ammonium nitrate and calcium ammonium nitrate.
Tier 1 substances require a licence for all purposes, including possession, use, acquisition, supply or import into Northern Ireland. It does not matter whether a person is acting for business or private purposes, a licence is required in order to deal with these substances. Individuals with a licence have special obligations towards keeping records and obtaining police consent for transactions over a certain weight or measure.
Tier 1 substances have to be kept sufficiently secured as determined by the specific circumstances in each case, and in particular by the quantity and nature of the substance concerned.
If a site contains 25 tonnes or more of ammonium nitrate or mixtures containing ammonium nitrate where the nitrogen content exceeds 15.75% of the mixture by weight, The Dangerous Substances (Notification and Marking of Sites) Regulations (Northern Ireland) require the person in control of the site to notify the enforcing authority; this is usually the Northern Ireland Fire and Rescue Service Board and, for some of the regulations, the Health and Safety Executive for Northern Ireland.
It is possible that fire and rescue services may attend incidents where ammonium nitrate fertiliser is being stored illegally. This may be to avoid the legislation or reporting requirements, but it could also be linked to illegal activity, such as the production of improvised explosive devices.