Biomass allocation and growth of Scots pine, Pinus sylvestris L., of various sizes (height 0.03–20 m) and ages (1–151 years) were investigated in two infertile sites (raised bog and sand dunes) to determine relative nitrogen and phosphorus limitations on productivity and their interactions and size-dependent controls. Dry mass weighted average nitrogen (NW) and phosphorus (PW) contents were higher in P. sylvestris in sand dunes than in those in the raised bog, but PW/NWratios overlapped between the sites. Leaf dry mass ratio (FL) and leaf-area ratio (LAR) increased with NW, and FLincreased with PW. The relative growth rate (RG) was more strongly associated with PWthan with NW. The net assimilation rate per leaf dry mass (NARM) scaled positively with PWbut not with NW, demonstrating that the stronger effect of PWon growth was due to modified biomass allocation and physiology (RG = NARM× FL), while NWaffected growth via biomass allocation. Partitioning and growth characteristics were poorly related to the PW/NWratio. The overall decrease of growth in larger trees resulted from their lower LAR and FL. Increases in size further led to a lower NWbut higher PW. We conclude that optimum productivity at a given NWrequires a certain minimum PW, not a specific "non-limiting" PW/NWratio. While nutrients affect growth by changing biomass allocation and physiological activity, size primarily modifies biomass allocation.