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 (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 microgram 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 microgram g(-1), R2 = 0.43), Bray No. 2 P (range: 0.1-15 microgram g(-1), R2 = 0.30), acid-extractable P (range: 0.6-11 microgram 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.