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Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco] is the predominant forest plantation species in the Pacific Northwest (PNW), with site productivity and fertilizer response influenced by climate and soil variations. This study investigates the utility of in situ 12‐week supply measurements of nitrogen (N), calcium (Ca), and phosphorus (P) to ion‐exchange resins (specifically Plant Root Simulator [PRS] probes) to estimate carbon (C):N ratios, soil nutrient contents (0–1 m), foliar nutrient concentrations, Douglas‐fir productivity (site index and basal area mean annual increment), and fertilizer volume response. PRS nutrient supply rates were correlated with N, Ca, and P soil nutrient contents (0–1 m), C:N ratios, and foliar nutrient concentrations. Low PRS NO3 supply rates (<25 mg N·m−2·burial period−1) were correlated with lower Douglas‐fir productivity and greater fertilizer volume response. PRS NO3 supply rates performed as well as total soil N contents and foliar N concentrations at estimating volume growth response to fertilizer. Twelve weeks after fertilization, PRS NO3, NH4, and Ca supply rates were significantly elevated compared to the unfertilized treatment. This research found that PRS probes were an effective in situ tool and are recommended for understanding N, Ca, and P nutrient availabilities, site productivity, and fertilizer response in Douglas‐fir plantations and for developing fertilizer prescriptions. Core Ideas: Plant Root Simulator (PRS) ion‐exchange resins estimated N, Ca, and P availability in Douglas‐fir forests.Soils with low PRS NO3 resulted in lower Douglas‐fir site index and basal area mean annual increment.Greater fertilizer response of Douglas‐fir was found on soils with low PRS NO3 and soil total N.Urea fertilization increased PRS NO3, NH4, and Ca supply rates after 12 weeks.In a subset of installations, PRS NO3 and NH4 were reduced to control levels after 32 weeks. [ABSTRACT FROM AUTHOR]