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What is thermal cycling?
Thermal cycling is a phenomenon caused by the repeated alternation between high daytime temperatures and cooler night-time temperatures. When ammonium nitrate granules repeatedly pass through their thermal transition point, they undergo volume changes of approximately 3–4%.
Over time, these repeated changes weaken the structure of the granules and impair their physical properties.
Consequences for fertiliser quality
Thermal cycling can lead to:
• Breakdown of granules
• Dust formation
• Increased porosity
• Caking and lump formation
• Reduced spreading performance and application quality
Beyond its impact on agronomic performance, dust formation and a reduction in product density can increase the sensitivity of fertilisers to detonation. Careful management of storage conditions is therefore essential for the safety of storage and distribution facilities.
Good storage practices
To minimise the risks associated with thermal cycling, it is recommended to:
• Store fertilisers in enclosed buildings, or in open structures that are properly ventilated
• Protect products from both direct and indirect exposure to sunlight
• Use shaded areas for outdoor storage whenever possible
• Cover products with a light-coloured tarpaulin where natural shade is unavailable
• Maintain an air gap between the tarpaulin and the big bags using pallets to promote ventilation
• Store big bags on pallets to prevent direct contact with the ground
The following situations can significantly increase the risk of thermal cycling:
• Direct exposure of big bags to solar radiation
• Storage in south-facing buildings with insufficient airflow
• Transparent doors or wall panels that increase heat build-up
• Storage on surfaces that may damage packaging
• Lack of protection against temperature fluctuations
Preparing for heatwaves
As heatwaves become more common, controlling storage conditions is a key factor in preserving the quality of nitrogen fertilisers and ensuring their safe use. Particular attention should be paid during the summer months and during prolonged periods of extreme heat, when the risk of thermal cycling increases significantly.
