While numerous studies have demonstrated the essential role of magnesium in the normal development of plants, the information as to the soil conditions governing the availability of this nutrient is relatively limited. In general it is considered that a true lack of magnesium in the soil, such as might result from leaching or overcropping, is the main cause of magnesium deficiency in plants. This aspect of the problem has received considerable attention (Imp. Bureau, 1940). It has become evident in recent years, however, that the absorption of magnesium by plants may be depressed under the influence of an excess of other nutrients, notably potassium and calcium, in the soil. In some instances this has been noted to result in the onset of magnesium deficiency in plants, despite an apparently adequate concentration of this element in an easily available form in the soil as shown by chemical methods.

The literature having a bearing on the inducement of magnesium deficiency in this manner has been reviewed by Walsh & Clarke (1944). These workers, from a study of magnesium deficiency chlorosis in tomatoes, showed that it was induced by heavy dressings of potassium causing a depression in magnesium uptake. While the effect of potassium alone in this respect was sufficient to induce the deficiency, it was also found that the sulphate radical played a prominent part. Where potassium was added other than in the form of the sulphate salt, the onset of the chlorosis was retarded and the symptoms were reduced in severity.