n Southern African Forestry Journal - Site and stand age effects on fertiliser responses in pulpwood plantations in Swaziland : scientific paper




Six fertiliser trials were established in separate stands in their 4<sup>th</sup>, 7<sup>th &lt;/sup&gt; or 12&lt;sup&gt;th&lt;/sup&gt; year on both granite and gabbro derived soils in the Usutu Forest, Swaziland. All trials compared the same factorial combination of N, P and K fertilisers. Tree growth response to fertiliser was measured five years after application. The trials demonstrate that meaningful responses to fertiliser can be obtained when applied to established pulpwood stands without thinning. Both stand age at application and soil parent material were found to influence response to fertiliser. With K, positive responses were obtained in the 3 and 6 year old stands but negative responses were evident applied to 11-year-old stands. Positive responses to K were also more pronounced on gabbro derived soils. Tree growth was improved by P applied in 6- and 11-year-old stands and by N applied in 11-year-old stands. Results are discussed in relation to nutrient cycling, stand development, soil fertility and records of long-term productivity at Usutu. It is proposed that observed declines in second rotation productivity in block A of the forest are associated with gabbro-derived soils and that this decline can be corrected by fertiliser application.


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