Soil phosphorus buffering measures should not be adjusted for current phosphorus fertility
L. L. Burkitt A D , P. W. G. Sale B and C. J. P. Gourley CA Tasmanian Institute of Agricultural Research, University of Tasmania, PO Box 3523, Burnie, Tas. 7320, Australia.
B Department of Agricultural Sciences, La Trobe University, Bundoora, Vic. 3086, Australia.
C Department of Primary Industries, RMB 2460, Ellinbank, Vic. 3821, Australia.
D Corresponding author. Email: Lucy.Burkitt@utas.edu.au
Australian Journal of Soil Research 46(8) 676-685 https://doi.org/10.1071/SR06126
Submitted: 14 September 2007 Accepted: 28 July 2008 Published: 2 December 2008
Abstract
Soil phosphorus (P) sorption is an important and relatively stable soil property which dictates the equilibrium between sorbed and solution P. Soil P sorption measures are commonly adjusted for the effect of current P fertility on the amount of P a soil sorbs. In the case of highly fertilised agricultural soils, however, this adjustment is likely to be inappropriate as it may mask changes in a soil’s capacity to sorb P, which could affect future P fertiliser applications. A study was undertaken to compare adjusted or unadjusted methods of measuring P sorption using 9 pasture soils sampled from southern Victoria which had previously received P fertiliser and lime. The P sorption assessment methods included: P sorption isotherms, P-buffering capacity (PBC) measures (slope between equilibrium P concentration of 0.25 and 0.35 mg P/L), and single-point P-buffering indices (PBI), with methods either adjusted or unadjusted for current P fertility. A single application of 280 kg P/ha, 6 months before sampling, resulted in a general negative displacement of unadjusted P sorption isotherm curves, indicating reduced P sorption on 8 of the 9 soils. Adding the Colwell extractable P concentration to the amount of P sorbed before calculating the slope (PBC+ColP), tended to negate this fertiliser effect and, in 2 of the 9 soils, resulted in a significant increase in PBC+ColP values. Increasing rates of P fertiliser application (up to 280 kg P/ha) resulted in a consistent trend to decreasing PBI values (unadjusted for Colwell P), which was significant at 4 of the 9 sites after 6 months. However, only minimal changes in PBI values were determined when PBI was adjusted for current P fertility (PBI+ColP). Phosphorus sorption properties appeared reasonably stable over time, although 2 soils, both Ferrosols, indicated significant linear increases in PBI values when these sites remained unfertilised for 30 months. Lime significantly increased both PBI and PBI+ColP values at all sites 6 months after application, but the effect generally diminished after 30 months, suggesting PBI measurements should not be taken immediately after liming.
These results demonstrate that unadjusted measures of P sorption are more likely to accurately reflect changes in soil P sorption capacity following P fertiliser applications and suggest that the unadjusted PBI be used in commercial soil testing rather that the currently adjusted PBI+ColP.
Additional keywords: single-point P sorption index, P buffering index, P sorption curve, single superphosphate, triple superphosphate.
Acknowledgments
The authors sincerely thank Bernie Watt, Beverley and Bill Lyons, Lynette and Chris Vaughan, Gwen and Barry Gilbert, Dorril and Kevin Beveridge, Brian Gleeson and family, Shirley and Colin Irvine, Josie and Neil Black, and Rosemary and Pat Roache for the generous use of their properties in this field study. We are extremely grateful for the laboratory assistance of Kelly Thompson and dedicated statistical assistance of Murray Hannah (Department of Primary Industries, Ellinbank) and Dr Scott Foster (Tasmanian Institute of Agricultural Research), and Graeme Ward for assisting in identifying suitable sites for this field study. This project was a component of the Phosphorus for Dairy Farms project and was funded by Pivot Ltd, Dairy Australia Ltd, the Department of Primary Industries Victoria, La Trobe University, and the University of Melbourne.
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