Effects of residual phosphorus and potassium fertiliser on organic matter and soil nutrients in aPinus patulaplantation

Summary On the Usutu plantation the sustained production from successive Pinus patula rotations has been a focus of active research for nearly 50 y. On 13% of the plantation, underlain by gabbro rocks, a 20% growth decline was reported between first (1R) and second (2R) rotations as a result of developing phosphorus (P) and potassium (K) deficiencies. The operational application of fertiliser containing 75 kg Pha−1 and 75 kg K ha−1 corrected the decline in yield. In 1989 P and K fertiliser was randomly applied, at the above rate, to one half of each of a number of trial plots in a 6-y-old, 3R crop. A 4R trial was re-established on the exact location of the former 3R trial during 1999. In 2004 intensive sampling was conducted to ascertain the influence of the residual fertiliser on the nutrient content of the foliage, forest floor and topsoil in the 4R crop. Fifteen years after the initial fertiliser application, 0.5 M H2SO4 extracted Pinorganic + Porganic in the topsoil was still 41 kg ha−1 higher in the fertilised plots than in the control plots. The total forest floor mass was not affected by the residual fertiliser, although the mass of the partially and fully decomposed forest floor litter increased by 3.5 t ha−1 (10% level). The residual fertiliser increased the P concentration in only the various needle components and had no effect on any other components or nutrients in the organic matter. We conclude that residual P fertiliser had a greater effect on foliar, forest floor and soil nutrient content than residual K fertiliser.