Soil Analyses as Indicators of Phosphorus Response in Young Eucalypt Plantations

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 (CaCl 2 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 CaCl 2 ‐extractable P (range: 26–162 μg kg −1 , R 2 = 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 , R 2 = 0.43), Bray No. 2 P (range: 0.1–15 μg g −1 , R 2 = 0.30), acid‐extractable P (range: 0.6–11 μg g −1 , R 2 = 0.37), and total P (range: 0.038–3.5 mg g −1 , R 2 = 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.