Nutritional diagnoses in loblolly pine (Pinus taeda L.) established stands using three different approaches

Investigation of the merits of different approaches for making diagnoses of nutrient deficiencies from the same dataset are rare. Vector analysis (VA), diagnosis and recommendation integrated system (DRIS) and critical level (CL) approaches were used for identifying stands deficient in N and P by means of foliar analysis in 60 managed stands of loblolly pine (Pinus taeda L.) in Southeast Texas. The study design was a randomized complete block with three replications of four treatments. The uniform set of four treatments: no fertilizer (control), elemental nitrogen (N) at the rate 224 kg/ha, elemental phosphorus (P) at the rate 112 kg/ha as urea and TSP, and a combination of N and P, were randomly assigned to trees within each subplot. Before fertilization and a year following treatment, current-year needles were sampled from the upper one-third of each crown and were analyzed for N, and P to estimate the potential for fertilizer response in each stand. Two years following treatment, nine tree heights per plot were measured, and volumes were calculated using tree dbh, average height, and the volume equation: V = 0.002668 (d^2h) - 0.13681. Initial volume was compared to actual volume growth response and prediction accuracy was calculated. The study revealed that the stands were deficient in one or both nutrients. There were significant percentage of loblolly pine stands that responded to fertilization and growth in deficient stands responded significantly to fertilization. The three diagnostic efficacies differed in predicting whether or not a stand would respond to fertilization, and no method was accurate for predicting response across soil groups. Soil group alone was a reasonable predictor of response to fertilization. Our study revealed that the three methods of interpretation have their advantages and work best when used together. However, it is vital to understand the limitations of foliar diagnosis regardless of the accuracy of the diagnostic technique employed.