Long-term availability of manganese to plants in soil-less potting media

Abstract

Relatively minor amounts of manganese (Mn) were lost in drainage waters from peat, ground Pinus radiata bark and eucalypt sawdust-based potting media, and Mn added via two oxides, two soils or MnSO 4 under conditions typical of those in nurseries. Losses were more substantial from these media after they had been heated at 110°C for 1 hour. The 10 mg kg −1 of native Mn in a Pinus radiata bark medium was sufficient for normal growth of Hebe ‘Inspiration’ and New Zealand Cape oats for 16 months. However, the 4.6-5.8 mg kg −1 of native Mn in peat and sawdust-based media was insufficient for 12 months of symptom-free growth. Amendment of the peat and sawdust media with 20 mg kg −1 Mn, as MnSO 4, allowed normal growth for 16 months. DTPA and EDTA were better able to assess plant-available Mn in the media than were Double Acid, 0.5 M NH 4OAc (pH 4.65), 0.5 M Ca(NO 3) 2 or 0.5 M Mg(NO 3) 2. A 1:1.5 (v/v) 2 mM DTPA extract of these soil-less media at planting had to contain at least 1.6 mg l −1 Mn to ensure an adequate supply for 12 months of normal growth of the test plants. Caution is needed in extrapolating these data to other media and plants.