The 3-PG (Physiological Principles in Predicting Growth) radiation-use efficiency model has been widely used and tested for predicting the primary productivity of forests all over the world. This radiation-use efficiency model accounts for plant nutrition through a user-defined dimensionless fertility parameter (f N) that determines the effects a unit of radiation. Currently, this fertility parameter has to be entered by the user based on intuition or experience. The goal of this study was to propose a fertility modifier (f N) for the radiation-use efficiency family of models based on soil chemical and physical variables. We determined gross- and net-primary productivity for a set of 10 intensively measured mini-plots of Pinus radiata D. Don on the South Island of New Zealand and then fitted a fertility modifier, f N, to the set of plots using a simplified radiationuse efficiency model. Fitted f N values were correlated to soil physical and chemical variables. The nutritional modifier, f N, significantly increased with the soil N (%) and decreased with the soil C:N ratio, and both, soil N and the C:N ratio, were measured in the upper 10 cm of soil (f N = 1.32 – 0.04 C:N + 0.99 N, r2 = 0.73, P = 0.009). If confirmed, this relationship may prove useful to estimate the fertility modifier of radiation-use efficiency models (e.g., 3-PG) for Pinus radiata plantations in New Zealand. However, caution should be exercised for sites where mineral nutrients other than nitrogen limit productivity.