Depending on root depth and the vertical distribution of water, the soil contributes to satisfy the transpiration demand of grass crops. Using soil humidity measurements repeated in time at different depth, the depth of water extraction (DWE) of a species can be measured in pure swards. However, that may be irrelevant in mixed swards because (i) roots of all species can be found in all horizons and (ii) the relationship between root density, soil humidity and water extraction is complex. To date, the use of natural abundance of (18)O is the only way to precisely compare the DWE of plants sharing the same ground. The work reported here describes the results obtained in a mixture of Dactylis glomerata, Lolium perenne, Festuca arundinacea grown in dense sward and believed to exhibit various abilities to exploit soil resources. Pure stands were also analysed and compared to the mixed crop. The results showed that when the soil profile exhibited a monotonous gradient of natural (18)O abundance in water, the ranking of the DWE of three species was possible and explained the differences in plant water status observed in summer.