Response of densely stocked loblolly pine (Pinus taeda L.) to applied nitrogen and phosphorus

Forest productivity may be enhanced by satisfying the nutrient requirement of trees through fertilization. Afield experiment was designed to study the response of densely stocked 1-yr-old loblolly pine to nitrogen and phosphorus in the presence of applied K on a Beauregard silt loam soil (fine-silty, siliceous, thermic Plinthaquic Paleudults) of the Gulf Coastal Plain. The continuous function design with three replications was used. Fertilizer P rate was based on the P adsorption characteristics of the soil. After 1 yr of growth, the response variables: height; total aboveground dry matter;foliage and wood dry matter, diameter at ground level; and N, P, and K contents were related to the rate of N and P applied by quadratic models with response surface methodology. The lack-of-fit test showed that the quadratic model was adequate in explaining the relationship between Nand P applied and all the response variables except height. Fertilization with N alone on this soil had a depressing effect on yield. Growth and nutrient uptake were enhanced when both Nand P were applied. The predicted N and P levels that would give maximum growth were beyond the rates of N and P tested, indicating that more time should be given to observe the full response.