A solid substantially water-soluble adjuvant composition 32 comprising a surfactant 24 and a particulate nitrogen-based plant nutrient 12. The surfactant 24 is supported on the particulate plant nutrient 12 which has been pre-milled to a suitable particle size. The surfactant 24 is optimised to a viscosity of less than about 100 mPas at 20 degrees Celsius. The invention may further comprise the addition of acidifier 26 to the composition, the viscosity of the acidifier and the surfactant having been optimised to produce the desired solid water-soluble adjuvant composition 32. A method 10 is also described which involves the milling 14 of a particulate nitrogen-based plant nutrient 12 which is then mixed with surfactant 24 and/or acidifier 26. The viscosity of the surfactant and/or acidifier are chosen or adjusted to appropriate levels to produce an optimum adjuvant composition.