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 soil and plant K and to identify critical soil and plant concentrations for grass yield. Maximum dry matter yield was obtained at a soil exchangeable K concentration (Kex) of 114 mg/kg dry soil (45 μм-K in the soil solution). Grass well-supplied with K maintained an average concentration of c. 200 m м-K in the tissue water of whole shoots in spring, although individual measurements varied in the range 140–320 mм, depending largely on the soil water availability. These tissue concentrations were not exceeded in soils with Kex values up to 800 mg/kg. Yield was reduced by 60% on plots low in K (Kex in the range 60–90 mg/kg). The yield reductions were probably due to low turgor because tissue osmotic pressures were low and potassium concentrations in tissue water (which averaged 64 mм) were probably too high to disrupt the biochemical functions of K. The results suggest that there is no universal critical tissue water concentration for K in the biophysical role of turgor maintenance, as other cations and solutes are able to substitute 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 universal critical osmotic pressure can be expected however, as this will depend on the availability of water and therefore on growing conditions.