Site and stand age effects on fertiliser responses in Pinus patula pulpwood plantations in Swaziland

Abstract

Six fertiliser trials were establishedin separate stands in their 4^th , 7^th or 12^th year on both granite and gabbro derived soils intheUsutu Forest, Swaziland. All trials compared the same factorial combination ofN, 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 whenapplied 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 ll-year-old stands and byN applied in Ll-year-old stands. Results are discussed in relationto nutrient cycling, stand development, soil fertility and records oflong-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 byfertiliser application.