**1.6 FeEDDHA based fertilizers**

86 Soybean Physiology and Biochemistry

Fig. 3. Hydrolysis species of Fe(3+) in equilibrium with soil-Fe (pKsol = 39.3; I = 0.03 M), after

Fe fertilizers can be administered through trunk injection, foliar application, and soil application. Trunk injection is expensive and only suitable for trees. Foliar application does not provide full control of Fe chlorosis, but can be useful as complementary technique next to soil application (Alvarez-Fernandez, et al., 2004). Soil application is the most common technique to manage Fe deficiency in soil grown crops (Lucena, 2006). The technique is based on increasing the Fe concentration in soil solution. On calcareous soils, soil application of Fe fertilizers based on organic Fe salts, Fe complexes of lignosulfonates, citrates, gluconates, and synthetic Fe chelates of limited stability (e.g. FeEDTA, FeDTPA and FeHEDTA) has limited or no result, because these fertilizers are not able to maintain Fe in soil solution. Only Fe chelates of higher stability (FeEDDHA and derivatives, with phenolic functional groups) are effective and provide the most efficient treatment to control Fe

Fe deficiency chlorosis is a persistent, yield-limiting condition for soybean (*Glycine max* (L.) Merr.) production in regions with calcareous soils (Inskeep and Bloom, 1986). In the North Central U.S., Fe deficiency is responsible for an estimated loss in soybean grain production of \$120 million per year (Hansen et al., 2004). Foliar Fe treatments and soil application of Fe chelates can be efficient in alleviating Fe deficiency chlorosis in soybean. However, in agricultural practice, these methods are only economically feasible for high-value crops and

Lindsay (1979).

deficiency (Lucena, 2006).

**1.5 Fe deficiency in soybean** 

not for soybean (Fairbanks 2000).

FeEDDHA is the iron(3+) complex of the chelating agent EDDHA, which is an acronym for ethylene diamine di(hydroxy phenyl acetic acid). EDDHA is also referred to as EHPG (ethylenebis-(hydroxy phenyl glycine)). This chelating agent was first synthesized by Kroll, introduced in 1955, but only fully described in 1957 (Kroll, 1957; Kroll, et al., 1957; Wallace, 1966). FeEDDHA was quickly recognized as very effective in correcting Fe chlorosis under soil conditions, also in comparison to other chelating agents (Wallace, et al., 1955; Wallace, 1962). The Fe3+ ion is bound by 2 carboxylate groups, 2 phenolate groups and 2 secondary amine groups in an octahedral complex of high stability with an intense red colour at neutral pH. The FeEDDHA complex owes its high stability in comparison to FeEDTA or FeDTPA complexes to the Fe-O (phenolate) bonds.

The current synthesis pathway for manufacturing EDDHA on an industrial scale is a Mannich-like reaction between phenol, ethylenediamine and glyoxylic acid. This reaction produces a mixture of 1) positional isomers, 2) diastereomers and 3) polycondensates, because 1) the reaction pathway allows for aromatic substitution in (o) ortho and (p) para position, 2) two chiral centers are introduced into the molecule leading to (R,R); (R,S); (S,R) and (S,S) configurations, and 3) undesired addition reactions take place between reactants and half products. The composition of the mixture of reaction products can be steered. After the reaction is terminated, an Fe salt is added to the reaction products to form Fe chelates.

Commercial FeEDDHA formulations can be operationally divided into 4 groups of compounds:


In this chapter, these 4 groups will be referred to as the FeEDDHA components. In commercial FeEDDHA formulations, the sum of the racemic and meso o,o-FeEDDHA content is referred to as the o,o-FeEDDHA content of the product. Generally racemic and meso o,o-FeEDDHA are synthesized in a ratio close to 1.

Racemic and meso o,o-FeEDDHA are diastereomers; the chelated Fe is bound by the same functional groups, but the geometry of the chelate differs: in racemic o,o-FeEDDHA, both phenolic rings are in equatorial position, while in meso o,o-FeEDDHA one phenolic ring is in equatorial and the other in axial position (Figure 4a and 4b). Due to the difference in geometry the amount of strain on the bonds with Fe differs, which is reflected in a higher complexation constant for racemic o,o-FeEDDHA.

The position of the hydroxyl group on the phenolic ring affects the complexation constant of FeEDDHA components more strongly than strain: in para-position the hydroxyl group is sterically inhibited from contributing to binding Fe. As a consequence, o,o-EDDHA binds Fe more strongly than o,p-EDDHA (see Table 1.1), which in turn binds Fe more strongly than p,p-EDDHA. Rest-FeEDDHA is a very heterogeneous group, comprising of compounds that vary in molecular weight, number of functional groups, etc, and hence also in complexation constant.

Fig. 4. Spatial structures of the FeEDDHA components **a)** racemic o,o-FeEDDHA; **b)** meso o,o-FeEDDHA; **c)** o,p-FeEDDHA with OH on the coordination complex; and **d)** rest-FeEDDHA (one possible polycondensate) (Schenkeveld et al., 2007).

Racemic and meso o,o-FeEDDHA are diastereomers; the chelated Fe is bound by the same functional groups, but the geometry of the chelate differs: in racemic o,o-FeEDDHA, both phenolic rings are in equatorial position, while in meso o,o-FeEDDHA one phenolic ring is in equatorial and the other in axial position (Figure 4a and 4b). Due to the difference in geometry the amount of strain on the bonds with Fe differs, which is reflected in a higher

The position of the hydroxyl group on the phenolic ring affects the complexation constant of FeEDDHA components more strongly than strain: in para-position the hydroxyl group is sterically inhibited from contributing to binding Fe. As a consequence, o,o-EDDHA binds Fe more strongly than o,p-EDDHA (see Table 1.1), which in turn binds Fe more strongly than p,p-EDDHA. Rest-FeEDDHA is a very heterogeneous group, comprising of compounds that vary in molecular weight, number of functional groups, etc, and hence also in complexation

Fig. 4. Spatial structures of the FeEDDHA components **a)** racemic o,o-FeEDDHA; **b)** meso

FeEDDHA (one possible polycondensate) (Schenkeveld et al., 2007).

on the coordination complex; and **d)** rest-

o,o-FeEDDHA; **c)** o,p-FeEDDHA with OH-

complexation constant for racemic o,o-FeEDDHA.

constant.


Table 1. Complexation constants of FeEDDHA components.
