Phytotoxicity of several iron fertilizers and their effects on Fe, Mn, Zn, Cu, and P content of African marigolds and zonal geraniums

Abstract

Zonal geraniums (Pelargonium xhortorum) from seed and african marigolds (Tagetes erecta), which are known to be highly susceptible to Fe toxicity problems, were grown with 1, 2, 4, or 6 mM Fe from ferrous sulfate, ferric citrate, FeEDTA, FeDTPA, FeEDDHA, ferric glucoheptonate, or ferrous ammonium sulfate in the subirrigation solution. FeEDTA and FeDTPA were highly toxic to both species, even at the 1 mM rate. Ferrous sulfate and ferrous ammonium sulfate caused no visible toxicity symptoms on marigolds, but did reduce dry weights with increasing Fe concentrations. Both materials were slightly to moderately toxic on zonal geraniums. FeEDDHA was only mildly toxic at the 1 mM concentration on both species, but was moderately toxic at the 2 and 4 mM concentrations. Substrate pH was generally negatively correlated with geranium dry weight and visible phytotoxicity ratings, with the least toxic materials, ferrous sulfate and ferrous ammonium sulfate, resulting in the lowest substrate pHs and the chelates FeEDTA, FeDTPA, and FeEDDHA the highest pH. The ionic Fe sources, ferrous sulfate and ferrous ammonium sulfate, suppressed P uptake in both species, whereas the Fe chelates did not. Fe EDDHA should be considered as an effective and less toxic alternative for the widely used FeEDTA and FeDTPA in the production of these crops.