The interactions of major cations (K,Ca,Mg) are studied in Sorghum (Sorghum dochna F.); the nutritive solutions are deficient or not, in magnesium. The mineral contents of the plants grown on the reference solution are compared to those of two solutions, in which magnesium is replaced by calcium or potassium. The limit threshold of deficiency does not affect the yield of plants, but reduces strongly the content of magnesium. On the other hand, no variation of calcium is observed since the magnesium substitution is effected by potassium. The experiments do not clearly show a Mg/Ca interaction in Sorghum.