CRITICAL PLANT K-CONCENTRATIONS FOR GROWTH AND PROBLEMS IN THE DIAGNOSIS OF NUTRIENT DEFICIENCIES BY PLANT ANALYSIS

Ryegrass was grown in a glasshouse in pots of soil at a range of avail able K, either with or without added Na. Critical plant K concentratio ns for growth were related to different functions of K in plants. Crit ical K concentrations were 126 mM (1.9% in DM) for biophysical functio ns in low- Na soils 82 mM (1.3% in DM), for biophysical functions in h igh -Na soils, and 46 mM (0.8% in DM) for biochemical functions. The i mplications of the findings for plant testing and the benefits of expr essing plant K concentrations on a tissue water basis are discussed.