Most fertilizers have some tendency to form agglomerates (caking) during storage. This caking is usually effected by: chemical composition, particle structure, moisture content hygroscopic properties, mechanical strength, product temperature, storage time and pressure. The aim of this study is to provide fundamental information on the performance of POLY4, a trademarked granulated polyhalite fertilizer by Sirius Minerals Plc, under long term storage as a fertilizer and in NPK blends. Evaluation of POLY4 blends “caking” is proposed using accelerated caking tests. These accelerated caking tests are of short duration and for this reason can be used in fertilizer granulation plants on a quality control basis. Crushing and Dynamic Vapour Sorption (DVS) tests for individual granules, and caking and DVS tests for POLY4 blends were performed to monitor the behaviour of POLY4 within the various blends. The addition of POLY4 to the blends has a positive effect on the blends by reducing the caking tendency. It was also found that the sample with the largest amount of POLY4 (50%) has the largest creep rate value and longest estimated storage time approximately 300 months.