Drying method effects on extractable phosphorus levels in plant tissue

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Abstract
The level of soluble tissue phosphorus (PO4‐P) may be correlated with the plant P nutritional status, but the amount extracted depends upon dry matter losses or the amount of enzymatic or heat—induced hydrolysis of organic P compounds during sample drying. Alfalfa (Medieago sativa L) and sugarbeet (Beta vulgaris L) plant parts grown under low and high soil P conditions, were freeze‐dried or oven‐dried at 40, 56, 70, or 100 C. Total K, P, and 0.35 N acetic‐acid‐soluble P (measured as PO ‐P) were determined. Dry matter losses were 0, 6.5, 3.6, 5.5, and 4.9 percent for the respective drying methods. The total‐P values, once corrected for dry matter losses, were not affected by the drying methods. The corrected PO4‐P values were 0.15, 0.17, 0.16, 0.16, and 0.19 percent, while the (PO4‐P)/P values were 0.63, 0.69, 0.67, 0.68, and 0.78 for each of the respective drying methods. The potential utility of extractable PO4‐P in describing the plants’ P nutritional status will depend upon rigorous sampledrying techniques. Freeze‐drying was the most satisfactory method tested, since it resulted in the least dry‐matter loss and least organic P hydrolysis.