Effect of phosphorus and potassium fertiliser on tree growth and dry timber production of Pinus patula on gabbro-derived soils in Swaziland

A trial designed to determine the optimum timing and rate of application of phosphorus (P) and potassium (K) fertiliser, to mitigate a growth decline observed inP. patula stands located on the gabbro-derived soils (underlying 13% of the plantation area) of the Usutu Forest, was sampled at rotation age (15 years). The P and K fertiliser was applied in three quantities, namely 20/20, 40/40 and 80/80 kg P/K ha–1. The 40/40 and 80/80 quantities were applied (1) all at planting, (2) all after pruning at the age of five years, and (3) as a split application where 20 kg P and K per hectare was applied at planting as a spot application and the remaining fertiliser applied after pruning (at age five years) as a broadcast application. Tree survival was not affected by either the quantity or time of application of the fertiliser. The quantity of fertiliser applied had a greater effect on the increase in tree growth and dry timber production than the timing of the application. here were indications that the early fertiliser application produced more timber than the late fertiliser application. The application of 80/80 kg P/K ha–1 significantly increased the quadratic mean diameter at breast height (DBH) from 20.0 cm to 23.0 cm; mean tree height from 19.6 m to 20.4 m; and volume production by 83 m3 ha–1 (29%). These results support the current fertiliser prescription on previously unfertilised stands. Furthermore, the application of fertiliser resulted in a more uniform DBH distribution. Compared to the control, the application of 80/80 kg P/K ha–1 fertiliser decreased basic wood density by 3% (from 0.3718 to 0.3610 g cm–3). The positive volume growth response to the application of fertiliser outweighed the disadvantage of a slight decrease in basic wood density as 28 t ha–1 (26%) more dry timber was produced as a result of fertilisation. In order to maximise dry wood production per unit area, the application of PK fertiliser to previously unfertilised gabbro-derived soils at the current recommended rate should continue. The applicability of the findings from this study to other P. patula stands on soils originating from basic geologies (dolerite, diabase, basalts and other igneous rocks, having similar low P and K contents) in southern Africa should be investigated. Keywords: basic wood density, plantations, pulp wood, stand structure, time of fertiliser application, yieldSouthern Forests 2009, 71(3): 235–24