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ALLEVIATION OF INTERVEINAL YELLOWING IN LEAVES OF PETUNIA “ROSE FROST” IN COMPOSTED SPRUCE BARK
Dried leaf material from the above contained high Mn levels (at >200 mg/kg, compared to <100 mg/kg for those grown in peat) without visible toxic Mn deposits on the leaves, and no indication of low Fe levels to produce a deficiency symptom.
Compost analysis have consistently showed high Mn levels in the CSB ranging from 3 to 50 mg/litre substrate depending on batches.
The following experiment was conducted to alleviate the yellowing symptoms seen in the leaves above by comparing Petunia ‘Rose Frost’ in a peat compost mix as a control; in CSB with no added Mn; in CSB with added Mn (total CaCl2 extractable at 35 mg/l) with three levels of added Fe (0, 0.5 and 1.0 that of Mn). All the treatments were given basal CRF split into those liquid fed (twice weekly) with a complete nutrient solution with or without Fe at 0.6 mg/l.
The plants were assessed when flowering commenced.
The leaves of those plants in peat controls, with or without Fe in the liquid feed were of excellent commercial quality and did not exhibit any yellowing symptoms.
Those in CSB, with or without added Mn, were unacceptable with interveinal yellowing in the upper and lower leaves and significantly lower shoot dryweights compared with the controls.
Liquid feeding only slightly reduced symptoms.
However, the leaves of those plants in CSB with added Mn and Fe did not exhibit interveinal yellowing, or significantly differ from the peat controls and contained 60 percent lower Mn and Zn levels compared with those leaves of plants in CSB without the added Fe.
The incorporation of Fe in CSB has alleviated the yellowing problems brought about by high Mn in CSB by reducing the uptake of Mn and Zn by the leaves.