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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 soils' 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 prior to sampling, resulted in a general negative displacement of unadjusted P sorption isotherm curves, indicating reduced P sorption on 8 of the 9 soils. However, 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 for 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 two soils, both ferrosols, indicated significant linear increases in PBI values when these sites remained unfertilised for 30-months. Lime significantly increased both the PBI and PBI+ColP values on 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.

Item Type:	Article
Authors/Creators:	Burkitt, LL and Sale, PWG and Gourley, CJP
Keywords:	single-point P sorption index, P buffering index, P sorption curve, single superphosphate, triple superphosphate.
Journal or Publication Title:	Australian Journal of Soil Research
ISSN:	0004-9573
DOI / ID Number:	https://doi.org/10.1071/SR06126
Additional Information:	Copyright © 2008 CSIRO
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