The dry weights of foliage and roots (excluding stembases) were compared in a 33×22 factorial experiment with Lolium perenne L. and Trifolium repens L. grown in pots with three nitrogen (0–300 ppm N), phosphorus (0–300 ppm P), and soil moisture (pF 3.2, pF 2.4, and waterlogged) treatments, each duplicated. Root/shoot ratios were influenced significantly (P < 0.001) by N, P, and W (soil moisture), but these main effects were qualified by highly significant first-order interactions and second-order interactions which included N and P. The lowest R/S ratios occurred where N, P, and W were all non-limiting, and the highest where there was an imbalance of these factors. The explanation suggested is that a decrease in the availability of N, P, or W causes an increase in the relative weight of roots, but in the presence of N, P, or W deficiency an increase in any one of these factors causes a further increase in the R/S ratio by increasing the relative efficiency of the foliage.