All water-soluble salts and mixtures have characteristic critical humidities; it is a unique material property.
For instance, the critical relative humidity of ammonium nitrate decreases 22% with a temperature from 0 °C to 40 °C (32 °F to 104 °F).
Table 2 shows CRH data for two-component mixtures: Table 2: Critical relative humidities of mixtures of salts at 30°C (values are percent relative humidity).
As shown, the effect of salt mixing is most dramatic in the case of ammonium nitrate with urea.
This mixture has an extremely low critical relative humidity and can therefore only be used in liquid fertilisers (so called UAN-solutions).