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Greenhouse Evaluation of Four Phosphorus Soil Tests in Relation to Phosphate Buffering and Labile Phosphate in Soils¹

ABSTRACT

Four soil P tests — Olsen, Colwell, Bray, and Mehlich — were evaluated in a greenhouse experiment in relation to plant uptake, relative yield, labile P, and buffer capacity on 30 soils varying in pH from 5.4 to 8.1. The Olsen and Colwell tests were most highly correlated with plant uptake, and the Olsen and Bray tests with relative yield. When the soils were stratified into weakly, moderately, and strongly buffered groups, it was found that extraction of labile P was depressed by increasing buffer capacity most in the Bray test, followed by the Olsen test, and least in the Colwell test. The Bray test tended to overcompensate for the effects of buffering on plant uptake and therefore underestimated the amount of available P in strongly buffered soils, whereas the Colwell method tended to overestimate it in strongly buffered soils. The Olsen test appeared to be correctly sensitive to buffering and therefore gave the highest correlations with both plant parameters. The Mehlich test extracted large amounts of nonlabile P in soils with pH >6.0 and was therefore weakly correlated with plant uptake and relative yield. The results showed that in a successful soil test, increasing buffer capacity will depress the extraction of labile P in the same way as it depresses uptake by plants.

¹ Contribution from the New South Wales Dep. of Agric., Agricultural Research Centre, Tamworth, N.S.W. 2340, Australia.

² Senior Research Scientist, N.S.W. Dep. of Agric., Agricultural Research Centre, Tamworth, N.S.W. 2340, Australia.

Received for publication May 14, 1979. Accepted for publication January 22, 1980.

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