The form of the algebraic relationship between the caking of granular fertilizers and moisture content, storage pressure, temperature and time has been investigated using a laboratory caking apparatus. This relationship is used, together with further work on the caking of individual granules, to assess various mechanisms which have been proposed to account for the phenomenon of caking. The theory of intergranular crystal bridging is not supported. For ammonium nitrate prills it would appear that caking is brought about by a deformation process based on plastic creep: the prills are bound together by the pressure deficiency beneath a curved meniscus. In the case of granular compound fertilizers a second deformation process is possible; this is brought about by the effect of stress upon the solubility of the fertilizer salts.

The various potential anticaking additives are surveyed, and their mode of action reconsidered in the light of the proposed mechanisms of caking.

D C Thompson, ICI Agricultural Division, Billingham, UK.

67 pages, 24 figures, 2 tables, 44 refs.