GRASS YIELD IN RELATION TO POTASSIUM SUPPLY AND THE CONCENTRATION OF CATIONS IN TISSUE WATER

A long-term field experiment on permanent grass at Rothamsted (UK) was studied between 1988 and 1991 to determine the relationship between s oil and plant K and to identify critical soil and plant concentrations for grass yield. Maximum dry matter yield was obtained at a soil exch angeable K concentration (K(ex)) of 114 mg/kg dry soil (45 mum-K in th e soil solution). Grass well-supplied with K maintained an average con centration of c. 200 mm-K in the tissue water of whole shoots in sprin g, although individual measurements varied in the range 140-320 mm, de pending largely on the soil water availability. These tissue concentra tions were not exceeded in soils with K(ex) values up to 800 mg/kg. Yi eld was reduced by 60 % on plots low in K (K(ex) in the range 60-90 mg /kg). The yield reductions were probably due to low turgor because tis sue osmotic pressures were low and potassium concentrations in tissue water (which averaged 64 mm) were probably too high to disrupt the bio chemical functions of K. The results suggest that there is no universa l critical tissue water concentration for K in the biophysical role of turgor maintenance, as other cations and solutes are able to substitu te for it. The ultimate determinant of turgor-dependent yield loss is tissue osmotic pressure. The critical tissue osmotic pressure in grass shoots in a typical season at Rothamsted was c. 400 mosm/kg. No unive rsal critical osmotic pressure can be expected however, as this will d epend on the availability of water and therefore on growing conditions .