Nine‐year Response of Thinned Slash Pine to Nitrogen, Phosphorus, and Potassium

Abstract

AbstractDiagnostic procedures are needed for determining response of slash pine (Pinus elliottii Elngelm. var. elliottii) plantations on productive sites to N, P, and K fertilizer late in the rotation. A 25‐yr‐old slash pine plantation (site index = 21 m at 25 yr) in southwest Louisiana was thinned to a residual basal area of 18 m2 ha−1 and fertilized at rates of 0 to 448 kg N ha−1, 0 to 171 kg P ha−1, and 0 and 56 kg K ha−1 at selected levels of N and P. Separate experiments were established to determine response of stands and individual dominant and codominant trees to added nutrients. During 9 yr after application, N fertilizer stimulated diameter but not height or volume growth of stands. The effect of N on diameter growth decreased with increasing tree diameter at breast height (dbh) at age 25. Nitrogen had no significant effect on diameter, height, or volume growth of individual dominant or codominant trees. Phosphorus fertilizer significantly increased diameter, height, and volume growth of stands and individual trees. Regression analyses showed that diameter growth culminated with 110 kg added P ha−1 and volume growth culminated with 132 kg added P ha−1. Height growth increased lineally over the range of 0 to 71 kg added P ha−1. In otherwise comparable treatments, individual trees that were fertilized with 56 kg K ha−1 grew faster in diameter, height, and volume than those receiving none. Currently available diagnostic procedures would have identified the plantation as one that should respond to P and K fertilizer, but not to N.