The phenomenon of granular fertilizer agglomeration in storage, known as "caking", has been investigated and explained via a plastic creep-capillary adhesion model. Experimental investigations were undertaken using accelerated caking test equipment,, fitted with a displacement transducer. The shear stress required to break the resultant cake, the creep rate, and the voidage were calculated. The steady-state creep rate for nitrate-based NPK granular fertilizer increased as a function of storage pressure and followed a power law relationship. This indicated dislocation creep as the probable creep mechanism for granular fertilizer in accelerated caking tests. A capillary adhesion-agglomeration model was developed, which took into account the increased granule-granule contact area caused by plastic creep. The model was validated experimentally using a range of granular fertilizer with an acceptable correlation found between experimental and model data.