The significance of residual phosphorus and potassium fertiliser in countering yield decline in a fourth rotation of Pinus patula in Swaziland

Research into the sustained production from short rotations of Pinus patula on the Usutu plantation has shown that operational application of P and K fertiliser corrected the 20% growth decline that was observed between first and second rotations on about 13% of the plantation. In 1989 a trial was established in a third-rotation crop to determine if the benefit from residual fertiliser would allow a reduction in application to the next crop. During September 1999 a PK factorial treatment combination was applied to the fourth rotation with the residual PK effect occurring as a split-plot treatment. In 2004 and 2005, intensive sampling was conducted in the trial. The results indicated that in the presence of residual fertiliser, the reapplication of only K fertiliser increased volume by 27m(3) ha(-1) (21.5%) at age seven years. This was statistically similar to the 25.2m(3) ha(-1) (20%) increase from a combined PK application. From these early results it would appear that enough residual P fertiliser remained available to supply the early nutrient demand of the next crop. Although residual K fertiliser also affected volume growth and foliar nutrient levels, it was insufficient to supply the requirements of the next crop. We conclude that fertiliser applications to successive crops can be adjusted to allow for the benefit from residual P applied to the previous crop.