Effectiveness of Digested Sewage Sludge Compost in Supplying Nutrients for Soilless Potting Media1

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Abstract
Marigold was grown on 4 media treatments with different levels of composted digested sewage sludge. On each medium, a number of fertilizer amendment treatments were studied to evaluate compost as a replacement for the amendments ordinarily required for complete media. Media contained 0, 33, 67, and 100% sludge compost (by volume); the remainder was unfertilized Cornell mix (equal volume mixture of sphagnum peat moss and vermiculite). The recommended amendments for Cornell mix (N, P, limestone, and trace elements) were deleted one at a time, all, or all but N. Severe P deficiency of −P media was fully corrected by compost at all rates. Compost provided only part of the N requirement at 33%. All trace element requirements were supplied by compost, yet toxicities did not occur; compost-amended media pH was ≥ 6.7, which limited metal availability to plants. Soluble salts limited yield somewhat on all treatments containing 67 or 100% compost. Fertilization of 33% compost media with only KNO3 (recommended rate for Cornell mix) allowed plant performance equal to the complete Cornell mix. Compost supplied much higher amounts of Fe, Zn, and Cu than are ordinarily added to media, and corrected an apparent marginal Cu deficiency-stress of complete Cornell mix. Composted digested sewage sludge was found to be an effective ingredient for potting media for marigolds when N is supplied from chemical sources.

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