The paper presents, against the background of literature of the subject, results of studies, based on several plants, on the changes in the water stress index as a function of soil solution salinity. This allows, already at the design-operational stage, to optimize the calculations of the maximum permissible time intervals between successive irrigations. Relative biomass gains given in the figures as a function of salinity facilitate economical control of irrigation. The method of studies developed by the author can be also used for other plants to determine the water stress moisture content and the permissible crop yield decrease under the conditions of irrigation with salinated water.