Phytotoxicity of Several Iron Fertilizers and Their Effects on Fe, Mn, Zn, Cu, and P content of African Marigolds and Zonal Geraniums

Zonal geraniums (Pelargonium ×hortorum) from seed and african marigolds (Tagetes erecta), which are known to be highly susceptible to Fe toxicity problems, were grown with I, 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. Iron deficiency chlorosis is a common problem on container-grown plants due to poor soil aeration, low soil temperatures, high bicarbonate irrigation water, etc. (Korcak, 1987; Wallace and Lunt, 1960). However, applications of inorganic Fe fertilizers such as ferrous sulfate are often ineffective in correcting Fe chlorosis (Martens and Westermann, 1991). Several synthetic chelating agents with high affinities for Fe have been shown to be much more effective in correcting Fe chlorosis in a wide range of plants (Martens and Westermann, 1991). However, some of these products can be phytotoxic to plants, and marigolds and zonal geraniums are reported to be especially sensitive to Fe toxicity (Albano et al., 1996; Bachman and Miller, 1995). The purpose of this study was to determine the relative phytotoxicity of several commercially-available Fe fertilizers and their effects on the uptake of Mn, Zn, Cu, and P in seedling marigolds and zonal geraniums. Materials and Methods Seeds of ‘Atlantis Yellow’ african marigold and ‘Cardinal Orbit’ zonal geranium were germinated in Jiffy Germination Mix #901 using the nutrient solutions described above. Subirrigation trays were filled to a depth of ≈2 cm and the plants were allowed to absorb the solution for 1⁄2 h before the excess was poured off. A split plot design using 10 replicate pots per treatment was used. Plot treatments were randomized on the greenhouse bench and the experiment was repeated once for each plant species. Plants were rated subjectively by both authors (by consensus) for phytotoxicity severity on a 0 to 5 scale (0 = dead, 3 = moderate injury, 5 = no injury) and stems were cut off at the soil line for shoot dry weight determination. Leaf samples consisting of the youngest fully expanded leaves on each plant were collected for nutrient analysis. Leaf samples were digested using a modified Kjeldahl procedure (Hach et al., 1987). Samples were analyzed for Fe, Mn, Zn, and Cu using atomic absorption spectroscopy and for P using the ascorbic acid method (Greenberg et al., 1985). Plants were rated, sampled, and harvested when they reached a marketable size for a 10-cm pot. Marigolds were transplanted on 1 Aug. 2001 and harvested on 16 Aug. 2001 for Expt. 1 and were transplanted on 27 Aug. 2001 and harvested on 11 Sept. 2001 for Expt. 2. Zonal geraniums were transplanted on 28 Nov. 2001 and harvested on 26 Dec. 2001 for Expt. 1 and transplanted on 22 Jan. 2002 and harvested on 4 Mar. 2002 for Expt. 2. At harvest, the surface 2 to 3 cm of the 11.6 cm deep substrate column in each pot was removed to ensure that substrate samples were taken from the active root zone. Substrate samples were extracted with deonized water using the saturated media extraction method (Warncke, 1986). Substrate pH was determined on the extracted solution using a pH/conductivity meter at a standard 25 C. Substrate samples were pooled by treatment into three replicate samples. Substrate pH was assumed to be similar for marigolds grown with the same treatments. Data were analyzed by regression analysis, analysis of variance, or analysis of covariance, with mean separations by the Waller-Duncan k ratio method (SAS, SAS Inst., Cary, N.C.) Results and discussion For zonal geraniums, phytotoxicity severity increased with increasing Fe concentrations in the irrigation solution for all Fe sources (Table 1). Toxicity symptoms appeared as marginal necrosis, necrotic spotting, cupping of the leaves, or occasionally chlorotic stippling on leaves of all ages. Although there were differences in the levels of phytotoxicity for all sources between Expts. 1 and 2, FeEDTA and FeDTPA were highly phytotoxic in both experiments. Plants receiving 1 mM Fe from these sources were extremely stunted and necrotic, and no plants survived with 4 or 6 mM Fe from either source. Plants receiving ferrous sulfate, ferrous ammonium sulfate, or ferric glucoheptonate showed very little to moderate toxicity in both experiments. Plants receiving FeEDDHA showed moderate toxicity in Expt. 1, but very little toxicity in Expt. 2 for (Jiffy Products of America, Batavia, Ill.) in seedling plug trays having cells 2 × 2 × 4 cm deep. The plug trays were cut into blocks containing 10 seedlings each at the time when the first true leaf began to emerge for most seedlings. The blocks of plugs were set into plastic subirrigation trays and were irrigated as necessary by allowing the blocks to sit in a 1-cm-deep solution containing 100 mg N, 20 mg P, 100 mg K, 0.55 mg Mn, 0.56 mg Fe, 0.26 mg Zn, 0.06 mg Cu, 0.22 mg B, and 0.05 mg Mo/L from diammonium phosphate, potassium nitrate, ammonium sulfate, ferrous sulfate, manganese sulfate, zinc sulfate, copper sulfate, boric acid, and molybdic acid. In addition, the solutions applied to each plug block contained 0, 56, 112, 224, or 336 mg Fe/L (0, 1, 2, 4, or 6 mM Fe) from ferrous sulfate, ferric EDTA (Sigma Chemical, St. Louis, Mo.), ferric DTPA (Sequestrene 330, Becker-Underwood, Ames, Iowa), ferric EDDHA (Lidoquest, LidoChem Inc., Hazlet, N.J.)], ferric glucoheptonate (Florikan, Sarasota, Fla.), or ferrous ammonium sulfate (Sigma Chemical). Ferric glucoheptonate was not included in Expt. 2. Plugs were transplanted into 10-cm plastic pots using Pro Mix BX (Premier Horticulture, Red Hill, Pa.) 1 week after plants started receiving nutrient solutions. All plants were grown in an open sided greenhouse with ambient air temperatures averaging 30/21 C (day/night) and provided a maximum photosynthetic photon flux of 1350 μmol·m·s in Fort Lauderdale, Fla. Plants were subirrigated every other day HORTSCIENCE 39(3):595–598. 2004. Received for publication 25 Nov. 2003. Accepted for publication 23 July 2003. Florida Agricultural Experiment Station journal series R-09190. The authors wish to thank Anita Durden, Luci Fisher, and Susan Thor for their assistance in this study.