Loblolly Pine Stand Fertilization at Mid-rotation to Increase Small and Large Sawtimber Volume

A fertilizer trial was established in Georgia in a thinned loblolly pine stand in 2014. The experimental design was three replications in a randomized complete block design using four fertilizer treatments. The treatments included: (1) a full dose of nitrogen (N) and phosphorus (P), and potassium (K) applied on February 18-19, 2014 and ½ dose NP or NPK application on February 18-19, 2014 with second ½ dose applied in February 2017 - all fertilizer treatment plots had a herbicide application in September-October 2013 prior to fertilizer treatments; (2) a herbicide-only treatment; (3) NPK-only treatment; and (4) an untreated control. The dominant soil series on the tract, based on Natural Resources Conservation Service (NRCS) soil maps is Eustis, a somewhat excessively drained loamy sand formed in coarse-textured marine and fluvial sediments. This soil series may not support good loblolly pine growth without NP or NPK fertilization as indicated by the current growth rate on the site through the early to mid 20-yr-old age class.

The major objectives of the study are to: (1) quantify the magnitude and duration of wood volume response to the fertilizer treatments; (2) determine changes in product class distribution; (3) determine the cash flow and rate of return for each fertilizer combination compared to unfertilized control plots, and (4) discern if ½ dose applications of NP or NPK fertilizers, applied three years apart, enhance growth compared to a single NP or NPK treatment on these sandy somewhat excessively well drained soils. Baseline soil, foliage, tree measurements and one year post-application soil and foliage samples were taken in January 2015. Third-year tree measurements and the second of the ½ doses of NP and NPK were taken/applied in February 2017 and are being summarized. In an earlier study, fertilized loblolly pines responded dramatically to the one-time fertilizer application. The NP treatment resulted in US$1,301/A of wood and the NPK, sulfur and copper treatment produced $1,605/A of wood. These values were compared to the control (herbicide only) which only resulted in $890/A in the eight-year study period, showing gains from fertilization of $411 and $715/A.