The least hygroscopic fertiliser is

The least hygroscopic fertilizer is SSP.
SSP is the most popular phosphatic fertilizer after DAP as it contains 3 major plant nutrients i.e. Phosphorus, Sulphur and Calcium along with traces of other micronutrients.
SSP is a grey to brownish coloured material containing monocalcium phosphate and calcium in almost equal proportions. It is the least hygroscopic but needs moisture-proof packing to avoid lump formation during storage.