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DIVISION S-7—FOREST & RANGE SOILS

Soil Analyses as Indicators of Phosphorus Response in Young Eucalypt Plantations

^a Cooperative Research Centre for Sustainable Production Forestry and CSIRO Forestry and Forest Products, Private bag No. 5, Wembley, WA, Australia 6913
^b Cooperative Research Centre for Sustainable Production Forestry and CSIRO Forestry and Forest Products, GPO Box 252-12, Hobart, Tasmania, Australia 7001
^c Gunns Ltd., P.O. Box 63 Ridgley, Tasmania, Australia 7321
^d Dep. of Agricultural Science, University of Tasmania, GPO Box 252-54, Hobart, Tasmania, Australia 7001
^e CSIRO Forestry and Forest Products, Private Bag No. 5, Wembley, WA, Australia 6913
^f Institute of Land and Food Resources, The University of Melbourne, Royal Parade, Parkville Victoria, Australia 3052
^g Centre for Forest Tree Technology, P.O. Box 137, Heidelberg, Victoria, Australia 3084

* Corresponding Author (Daniel.Mendham{at}csiro.au)

The use of P fertilizer in Eucalyptus plantations can result in significant and economically viable increases in timber production. Soil analyses could potentially indicate sites requiring more P fertilizer than is currently applied and prevent excess P fertilizer application on sites where little or no response would be obtained. The ability of several soil P analyses to predict first year growth responses of E. nitens and E. globulus to P fertilizer was assessed in 24 previously established field experiments situated in southeast and southwest Australia on a range of soil types. Soil P analyses that were assessed included an intensity-based analysis (CaCl₂ extracted P), quantity-based analyses (total P, bicarbonate extracted P, and Bray No. 2 P), and quantity–intensity relationships (P adsorption curves). An excellent relationship was found between CaCl₂-extractable P (range: 26–162 µg kg^-1, R² = 0.83) and first year growth response to P applied at planting for 21 of the 24 field experiments. Quantity-based P analyses, such as bicarbonate P (range: 2–63 µg g^-1, R² = 0.43), Bray No. 2 P (range: 0.1–15 µg g^-1, R² = 0.30), acid-extractable P (range: 0.6–11 µg g^-1, R² = 0.37), and total P (range: 0.038–3.5 mg g^-1, R² = 0.39) did not correlate as well with plant growth response to P application. However, quantity-based P analyses may be useful if specific calibrations were developed for a limited range of soils. Inclusion of P adsorption data in multiple regressions of soil P concentration against relative yield generally did not improve the relationships. The excellent relationship between plantation response to P fertilizer and soil P intensity suggested that such analyses may be useful for managing P fertilizer in eucalypt plantations across the wide range of sites where eucalypts are currently being planted.

Abbreviations: EPC, equilibrium phosphate concentration • PBC, P buffer capacity