Growth response of four tropical plantation timber species to increasing phosphorus supply and assessment of phosphorus requirements using foliar analysis

An understanding of the comparative nutritional requirements of important timber species is fundamental in the development of sustainable systems for the production of wood from plantations on many infertile, highly weathered soils in the humid tropics. We established a field experiment comparing the responses of four highly-valued cabinet timber species (Cedrela odorata, Agathis robusta, Flindersia brayleyana and Castanospermum australe) to phosphate fertiliser on a soil very low in chemically extractable phosphorus (4 mg P/kg soil). Similar soils derived from Palaeozoic metasediments are widespread throughout the humid tropics of northern Australia, south-east Asia and the south-western Pacific, and are typical of those on many sites available for plantation forestry in the region. Phosphorus fertiliser treatments (ranging from 0 to 300 g P/tree) were applied soon after planting to all four species, as was a basal fertiliser dressing supplying adequate levels of all other mineral nutrients. Two species (F. brayleyana and C. australe) maintained good growth even without additional P and showed little or no response in growth or foliar P concentration to increased P supply. The other species (C. odorata and A. robusta) responded strongly in both growth and foliar P concentration to increasing P supply. In these responsive species, chemical analysis of foliage at 17 months after planting was shown to have potential diagnostic and prognostic value. Phosphorus concentrations less than 0.30% dry matter (DM) in the petiole plus rachis of young mature leaves of C. odorata, and less than 0.19% DM in distal needle leaves of A. robusta, we associated with diminished growth of these two species both at the time of sampling and 21 months later. The results indicate that there are significant differences in the ability of tropical tree species to acquire and use phosphorus, and consequently that simple prescriptive applications of phosphorus fertiliser to young plantations, irrespective of soil type or tree species, are often inefficient