Fertilizers with a lower critical relative humidity and higher hygroscopicity are the most susceptible to dampness and caking. In high-humidity environments, these fertilizers rapidly absorb moisture from the air; the surfaces of the granules dissolve and subsequently recrystallize, forming "salt bridges" that bind the particles into hard, solid lumps.
Highly Caking-Prone Fertilizers
Ammonium Bicarbonate: Highly hygroscopic. If left exposed to the air at room temperature for just one day, it will absorb significant moisture and deliquesce. This leads not only to rapid caking but also to substantial nitrogen loss-up to 77.25% of its nitrogen content can volatilize if left exposed for half a month at 20°C.
Ammonium Nitrate: Has a critical relative humidity of only about 50% and is extremely hygroscopic. If the packaging is damaged, it quickly absorbs moisture and cakes; in severe cases, it may even liquefy and flow, posing potential safety risks.
Calcium Nitrate: With a critical relative humidity of approximately 50%, it rapidly absorbs moisture whenever the ambient humidity exceeds this threshold. The granules easily stick together, making it one of the types of water-soluble fertilizers most prone to caking.
Moderately to Highly Caking-Prone Fertilizers
Superphosphate: Contains free acid and residual moisture, making it quite hygroscopic. Upon exposure to dampness, it quickly forms hard, solid lumps. Furthermore, its effective phosphorus content drops significantly after 170 days of storage.
Agricultural-Grade Potassium Chloride: While the pure substance is not highly hygroscopic, agricultural-grade products often contain impurities that cause them to gradually absorb moisture in high-humidity environments, leading to caking over time.
High-Nitrogen Compound Fertilizers: These often contain large amounts of highly soluble ingredients like urea or ammonium chloride. The granules easily bond together via "salt bridges," a process that is further accelerated when the fertilizer is stacked and subjected to pressure.
