Growth Responses of Loblolly Pine in the Southeast United States to Midrotation Applications of Nitrogen, Phosphorus, Potassium, and Micronutrients

Abstract

Growth of midrotation pine plantations in the southeast United States tends to be limited by nitrogen (N) and phosphorus (P). Routine applications of urea and diammonium phosphate ameliorate N and P deficiencies; however, questions concerning what other nutrients are likely to be limiting growth are being raised. Consequently, a trial series with 23 study installations was established in loblolly (Pinus taeda L.) stands, aged between 9 and 25 years, with the aim of determining whether stands would respond to potassium (K) additions once the N and P deficiencies were corrected and whether the application of a full suite of macro- and micronutrients would further increase growth. On average, N plus P applications resulted in a mean growth improvement over unfertilized controls of 3.71 m3 ha−1 year−1 for 8 years after fertilization. Further growth improvements in response to the application of K, either with the N and P, or together with a range of macro- and micronutrients, were found to be dependent on location. Studies located on Pleistocene terraces, between 10 and 65 m in elevation, associated with ancient sea levels including the Talbot, Penholloway, Wicomico, Sunderland, and Coharie terraces of Georgia and the Carolinas, showed a smaller than average positive response to the addition of N and P (2.66 m3 ha−1 year−1), with further increases in growth when K was applied as well (additional 1.33 m3 ha−1 year−1) and a further increase when a complete suite of nutrients was added (additional 2.59 m3 ha−1 year−1). Studies located elsewhere in the South showed an average response to the addition of N and P (mean improvement of 4.28 m3 ha−1 year−1), with no improvement in growth when additional nutrients were added. These results can assist foresters in identifying stands that are potentially responsive to applications of nutrients other than N and P.