Growth and foliar nutrient response to recent applications of phosphorus (P) and potassium (K) and to residual P and K fertiliser applied to the previous rotation of Pinus patula at Usutu, Swaziland

in a long-term phosphorus (P) and potassium (K) fertiliser trial, established on gabbro-derived soil in the Usutu plantation, the growth and foliar response of 6-year-old Pinus patula were studied. In 1989, when the third rotation (311) trees were 6 years old, 75 kg elemental P and K per hectare, respectively, was applied to one-half of 52 plots. In the fourth rotation (4R) a factorial design with applications of 0, 25 and 50 kg P and K per hectare was superimposed on the previous trial plots. The 50 kg ha(-1) application consisted of two applications of 25 kg ha(-1) at planting in 1999 and after pruning at age 5 years in 2004. The results indicated that on plots fertilised for the first time, the application of 50 kg K ha(-1) increased the foliar N, P and K concentrations and the application of 50 kg P ha(-1) increased foliar P and K concentrations. Volume increment over a 2-year period was increased by 7 or 8 m(3) ha(-1) when either foliar P was above 0.14% or K was above 0.50%. The maximum volume increment was achieved when both nutrients were at levels above their respective critical values. In the senesced needles, P fertiliser increased the P concentration, while K fertiliser increased the K concentration, which showed that the application of fertiliser reduced these nutrient deficiencies. There was evidence that the foliar nutrient response to residual P fertiliser was greater than the response to residual K fertiliser. These results suggest that fertiliser application to successive rotations can be adjusted to allow for the benefit of residual P fertiliser. The critical foliar values for P, K as well as the nutrient ratios can be used to monitor the response to revised fertiliser prescriptions. (C) 2008 Elsevier B.V. All rights reserved.