Soluble fertilizers are typically formulated with metal-aminopolycarboxylic acids [APCA (i.e., chelating agents)] of Cu, Fe, Mn, and Zn. These metal–APCA complexes, however, are also applied as single-metal chelate solutions to foliage, soil/substrate, or both for correcting micronutrient deficiency like Fe chlorosis (Mortvedt, 1991). Common chelating agents used in fertilizers include EDTA, DTPA, and EDDHA (Lucena, 2003). These chelating agents differ in stability/formation constants, i.e., the chemical bond-strength of the ion-ligand complex with metals as a function of pH, but they share a common trait; they are synthetically produced and not readily biodegradable (Borowiec et al., 2007; Sillanpää, 1997). In Europe, these chelating agents, especially EDTA, are persistent in the environment and are believed to have the ability to extract and mobilize heavy metals from sediments in surface and groundwaters; transporting extracted metals in the water column (complete review in Albano, 2011). For these reasons, in Europe, EDTA is being replaced where possible with biodegradable chelating agents like [S, S′]-EDDS (EDDS), which is a structural isomer of EDTA that is reported to have similar functionality as a chelate (Metsärinne et al., 2001; Neal and Rose, 1973). However, several other readily biodegradable “green” chelating agents are also being considered as replacements including methylglycinediacetic acid, L-aspartic acid N, N-diacetic acid, sodium diethanolglycine/2-hydroxyliminodiacetic acid, iminodisuccinic acid with salts, glutamic acid diacetic acid, and N-(1,2-dicarboxyethyl)-D,L-aspartic acid (Glauser et al., 2010; Lucena et al., 2008).
Little is known about the use of EDDS in horticultural crop production. Its use as an Fe-chelating agent in the production of marigold was reported by Albano (2011) and Albano and Merhaut (2012) where it was found that plants supplied with FeEDDS were not different in growth or foliar Fe concentration than plants supplied with FeEDTA, FeDTPA, FeEDDHA, or FeSO4. Leachate solution spectral properties and chemistry differed, however, between these Fe chelates. It was found that FeEDDS maximally absorbed at a shorter wavelength (238 nm) than either FeEDTA (258 nm) or FeDTPA (260 nm) and that Fe-catalyzed photodegradation of FeEDDS occurred at a rate close to twice that of FeEDTA when exposed to light (Albano, 2011). It was also found that FeEDDS leached less Fe and Mn than FeEDTA, FeDTPA, or FeEDDHA during the production cycle of marigold (Albano and Merhaut, 2012). Therefore, the broad objectives of this study were to gain knowledge on FeEDDS and EDDS interactions with peat-based substrate. Specific objectives were to 1) compare effects of Fe source (FeEDDS, FeEDTA, FeDTPA, FeEDDHA, and FeSO4); and 2) compare effects of chelate-ligand (EDDS, EDTA, and DTPA) on peat-based substrate pH and solubility of Cu, Fe, Mn, and Zn.
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