During the growth of underplant and cover crop interactions occur, the patterns and mechanisms of which are not totally known, particularly within the changeability of ecological factors. Joint use of environmental resources may modify the uptake of nutrients from the bedding and their accumulation in the particular plant organs. The aim of the study was the determination of the effect of interactions between spring barley and red clover on phosphorus content in their organs against diversified water provision. In a pot experiment (three cycles), the study factors were: (1) plant sowing type (growth of each species in pure and mixed sowing), and (2) plant provision with water (dose that satisfies plant requirements and one lowered by 50%). Mixture was composed according to the additive pattern. Phosphorus content was marked in plant material uptaken at five stages set by the developmental rhythm of barley in pure sowing in the conditions of more favourable water supply. Sowing type had no effect on phosphorus content in spring barley organs but mixed sowing resulted in the accumulation of a higher amount of phosphorus in the roots at the straw-shooting stage, and a lower amount in the shoots and spikes at the end of growth. In red clover, mixed sowing lowered phosphorus content in the roots at cereal maturity stage. Competition on the part of barley inhibited the uptake and accumulation of phosphorus by red clover. Water supply did not diversify phosphorus content in the organs of spring barley and red clover but water deficit almost throughout the entire period of growth limited phosphorus uptake by both species. Competition on the part of barley was for red clover a stronger factor that impeded phosphorus accumulation in plant organs than water deficit. Water stress did not intensify further the negative effect of competition.

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