Growth Response of Hydrilla to Extractable Nutrients in Prepared substrates

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Abstract
A new extractant consisting of 0.2N acetic acid, 0.25N ammonium nitrate, 0.015N ammonium fluoride, 0.012N nitric acid, and 0.002N ethylenediaminetetracetic acid was used to extract P, K, Ca, Mg, Fe, and Cu from a natural lake substrate and from artificially prepared substrates. The precision and effect of extraction time on nutrient yield were measured on the natural substrate. The growth response of hydrilla (Hydrilla verticillata, Royle) to concentrations of the nutrients estimated with the extractant and to N was measured. Extractable P and Fe increased with extraction time whereas extractable Cu decreased. Extraction time had no effect on extractable K and only the 5 min extraction time resulted in significantly lower yields of Ca and Mg. Hydrilla shoot weight increased linearly under laboratory conditions as nutrient concentrations increased in the rooting media, suggesting that growth was limited by nutrient availability to the roots. High correlation among the nutrients prevented determination of a single limiting nutrient. Concentrations of P and K in rooting media were positively related to concentrations in hydrilla tissue, but when there was 153 μg P/g and 8 μg K/g or more in the rooting media, P and K content of hydrilla tissue remained constant. Concentrations of N, Ca, Mg, Fe, and Cu remained constant throughout the range of these nutrients in the rooting media.