Available phosphorus in potting media: Extractants and interpretation of extract data

Abstract

Seven extractants—water and 2 mM DTPA at 1:1.5 by volume, saturated media extract with and without added DTPA, Mehlich double acid, Spurway, and NaHCO₃—were used to estimate the availability of P in organic potting media based on peat, sawdust, or bark, both with and without added soil. Extract results were compared with the foliar symptoms and growth of a range of plants which varied widely in their response to P. Inclusion of soil in the otherwise soilless media markedly decreased P extracted by all extractants except double acid and NaHCO₃, but even for them there was some reduction. Different soils decreased extractable P to different extents, but perlite, vermiculite, washed quartz sands, rice hulls, diatomite, and charcoal had no effect on extracted P when added at rates up to 40% by volume. Much of the P fixed by soils and not extracted by mild extractants was available to plants. The inclusion of soil had a more marked effect on extractable P than on P concentration in the shoots. Fe added as FeSO₄‐7H₂O reduced the concentration of extractable P in soilless media. Conversely, high levels of P added to these media induced Fe deficiency in some species.

Concentrations of P in all extractants were highly correlated with plant response if soilless and soil media were considered separately, but only for DA and, to a lesser extent, NaHCO₃, could the same interpretation criteria be used for both soilless and soil media.

Plant response to P added as single superphosphate ranged from acute deficiency in tomatoes to severe toxicity in Banksia ericifolia, Grevillea ’Copper Crest’, and Brachysema celsianum. The results indicate that, irrespective of which extractant is used, interpretation of test data must take account of widely different responses among plants to P in potting media. Some modifications to currently used interpretation criteria are suggested.