Low iron supply aggravates manganese toxicity in zonal geraniums growing in soilless potting media

Abstract

Zonal geranium (Pelargonium x hortorum) cultivars growing in soilless potting media based on Pinus radiata bark, and of pH 4.5–6.5, developed manganese (Mn) toxicity symptoms even though the medium contained only 26 mg‐kg^‐1 total Mn. The toxicity was partly or wholly alleviated by increased iron (Fe) supply. Ferrous sulphate (7H₂O) added to the medium before potting or weekly drenches with Fe chelates were effective, although the combination of the two tended to produce the best plants. Acidification of the medium caused by Fe‐stress is a major factor in mobilizing toxic levels of Mn from soilless media. While Fe supply had the major effect on the expression of Mn toxicity, this toxicity was produced in Pelargonium cultivars if water extracts (1:1.5 v/v) of the medium contained more than about 2 mg‐L^‐1 Mn.