AVAILABLE PHOSPHORUS IN POTTING MEDIA - EXTRACTANTS AND INTERPRETATION OF EXTRACT DATA

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.