## 1 Introduction
They first calculated fertilizer response (FER) by subtracting the ratio of biomass growth to initial biomass in fertilized plots from control plots. Sites with the lowest observed FER were given an arbitrary FR value of 0.6 and the sites with FER = 0 were given FR values of 1. This method of FR estimation requires fertilization trials on all the soil types in a region which is both costly and time consuming [10]. Site index (SI) is one of the most common measures of site quality because the height growth of dominant trees is less correlated with stocking than diameter growth and highly correlated with productivity [15]. [PERSON] _et al_. (2004) [16] used SI to estimate FR. However, they used FR as a tunable parameter to match the 3-PG predicted productivity with observed productivity. The USDA NRCS SSURGO database [17] contains information about SI of major soil series across the southeastern US. SI in the SSURGO dataset is defined as the average height that dominant and codominant trees of a given species attain in a specified number of years, and it is applied to well stocked and even-aged stands. SI is a realized measure of site quality and is widely used in many traditional growth and yield models as a driver of productivity [15]. SI is a function of soil fertility, soil moisture, and climate. Although there is a strong relation between rainfall and forest productivity, previous research has shown that soil fertility has a stronger impact on productivity of loblolly pine in the southeastern United States [18; 19; 20; 21; 22; 23; 24]. Within the natural range of loblolly pine, rainfall exceeds evapotranspiration [25]. Actual evapotranspiration values for loblolly pine in the southeastern US range from 570 to 1050 mm year\({}^{-1}\) while rainfall in this region ranges from 1050 to 1825 mm year\({}^{-1}\). Furthermore, results from a number of irrigation and fertilization studies show that the response to fertilization is much greater than irrigation. Studies with loblolly pine in the Sandhills of North Carolina [26] and lower Coastal Plain and Piedmont of Georgia [23; 27; 28] have shown that fertilization has larger impact on growth than irrigation in loblolly pine stands. In a study of fertilization and irrigation in loblolly pine by [PERSON] _et al_. (1998) [26] on an excessively drained, sandy soil in North Carolina, fertilization increased volume growth by 188% over four years comparison to control plots while irrigation only increased volume growth by 17%. In a similar fertilization and irrigation study in loblolly pine in the lower Coastal Plain of Georgia by [PERSON] _et al_. (2008) [28] revealed no significant effects of irrigation on height, basal area, and volume growth after six growing seasons, whereas fertilization significantly increased volume growth by 55% compared to control plots. In addition, recent work with rainfall exclusion by [PERSON] _et al_. (2015) [29] showed that an approximate 30% reduction in throughfall did not affect loblolly pine productivity in Florida and Virginia, whereas fertilization significantly increased productivity compared to nonfertilized treatment. Therefore, for loblolly pine in the southeastern US, we hypothesized that soil fertility is the main driver of productivity and that FR could be predicted from SI. The objective of this study was to use SI for the _a priori_ prediction of FR that could be used as a fixed rather than a tunable parameter in 3-PG\({}_{\rm{lob}}\)[6], a variant of 3-PG to simulate growth of loblolly pine across the southeastern US. This study also assessed the potential to use SI from the Soil Survey and Geographic Database (SSURGO) [17] to predict FR for individual soil series in a county.