Growth of Eucalyptus species in a Brown Kandosol, and changes in soil phosphorus fractionation following fertilisation

As observed with many soils, much of the P in the Brown Kandosol soils of Weipa (Australia) is associated with organic matter. However, following bauxite mining, much of this organic matter is either lost due to mineralisation, or is ‘diluted’ by the mixing of the soil profile. Using a sequential P extraction, the partitioning of P following fertiliser application was examined in an ‘undisturbed’ (Surface) soil and a Mixed soil. In addition, the effect of split-P applications on the growth of 2 native Eucalyptus species was examined. Following its addition to the soil, much of the P was converted comparatively rapidly to forms with reduced availability; by the time of the first measurement (4 weeks) only approximately 10% of the added P remained as the readily available AER-P. For the Surface soil, much of this added P was initially converted to organic P (Po) (measured as hydroxide-Po), before progressively moving into the hydroxide-Pi fraction. In comparison, in the mixed soil, competition for P from microbial biomass was lower (due to a lower organic matter content) and the P was rapidly converted to the hydroxide-Pi fraction before moving to unavailable forms (residual P). Although the use of split-P applications was expected to increase plant growth, maximum growth of Eucalyptus tetrodonta and Eucalyptus leptophleba was generally achieved when all P fertiliser was applied in the first few weeks of growth. Indeed, splitting the P application beyond 16 weeks caused a significant reduction in growth.