The effect of sodium on potassium nutrition and ionic relations in Rhodes grass

The effects of sodium on the response of Rhodes grass (Chloris gayana) to potassium have been studied in pot trials. The amount of potassium required to achieve maximum growth was progressively reduced as the level of sodium application was increased from 0 to 400 mg per pot. The concentration of potassium in the tops of plants growing at 95% of maximum yield was also progressively reduced from 2.7% in plants not fertilized with sodium to 0.5% in plants receiving 400 mg sodium per pot. It is concluded that such interactions between the levels of major elements preclude the use of a 'critical potassium level' for diagnosing potassium deficiency in Rhodes grass. An alternative diagnostic scheme which takes account of interactions between some of the major elements is suggested. This scheme requires that plants would be adequately supplied with potassium if two criteria were met simultaneously: the potassium concentration was above the minimum specifically required (0.5% of the dry matter for Rhodes grass) and the cation-anion balance was above a minimum level characteristic for that particular species (800 m-equiv./kg dry weight for Rhodes grass).