Growing ectomycorrhizal (ECM) plants in hydroponics is not common and probably not desirable, especially with fungal partners producing hydrophobic mycelia. The addition of a solid substrate with low buffering capacity to the cultivation system permitted growth of ECM Pinus sylvestris seedlings in a more root- and fungus-like environment. In such semihydroponic cultivation systems, both hydrophilic (Thelephora terrestris) and hydrophobic (Suillus luteus) fungi can grow well, provided the substrate is not continuously flooded. In the present investigation, P. sylvestris seedlings were grown at two suboptimal P addition rates. Mycorrhizal seedlings had significantly lower P contents in aboveground and higher P contents in belowground plant parts than non-mycorrhizal (NM) pines. When mycorrhizal plants are grown under steady-state conditions, the controlled addition of nutrients according to the Ingestad concept (Ingestad and Ågren 1995) does not take into account the nutrient requirements of the associated mycobiont. Therefore, the retention of nutrients in the mycelia can result in a decreased growth of mycorrhizal plants when compared to NM controls. Under steady-state conditions, plant and fungal development both reach an equilibrium sustained by feedback mechanisms in the allocation patterns. The maximal growth rate of different mycobionts does not necessarily occur at the nutrient addition rate resulting in maximal growth rate of a host plant. Ergosterol concentrations in roots and in growth substrate indicate that S. luteus grew more vigorously at the lower than at the higher rate of P addition.