Four processes may be involved in the reaction of Fe-chelates with soil that may reduce their presence in the soil solution:
• Fe-chelate or chelating agent degradation
• Replacement of the Fe by competing metals
• Sorption of the Fe-chelate onto the soil surfaces
• Percolation (lixiviation)
Chemical degradation of Fe-chelates is slow in the dark (Hill-Cottingham, 1955; Lahav and Hochberg, 1975). Thus, they have been considered as non-environmentally degradable, despite the fact that no experimental data are yet available proving an accumulation associated with agronomic practices. New biodegradable chelating agents (see EDDSA and IDSA in Figure 5-1) have been recently introduced to substitute EDTA in other industrial uses (Nowack and VanBriesen, 2005), and it may be possible to use them in Fe chlorosis correction.
The other three processes are also important in soils. Chelates and chelating agents may react with other metals or protons. Only adequate chelating agents may complex Fe in soil conditions (Lindsay, 1979; Lucena et al, 1988; Norvell, 1991; Yunta et al, 2003a; Sierra et al, 2004). Metal competition can be studied theoretically by means of chemical speciation programs, provided that thermodynamic data are available for all the possible reactions (Yunta et al., 2003b; Lucena et al., 2005). The early works of Lindsay (1979) are a good example. The theoretical stability of the most common commercial Fe chelates in soil conditions is shown in Figure 5-4. Only EDTA and analogous compounds are not stable enough to maintain chelated Fe in the soil solution. In the conditions found in soils at pH values below 9, which is a normal pH limit for agronomic practice, the stability of o,o-EDDHA/Fe3+, EDDH4MA/Fe3+, EDDH5MA/Fe3+ (the number in EDDHMA/Fe3+ describes the position of the methyl substitution in the aromatic ring) and EDDHSA/Fe3+ is sufficient to maintain all Fe in solution;
the stability sequence is, in decreasing order, EDDHSA/Fe3+ > EDDH4MA/Fe3+ > EDDH5MA/Fe3+ ~ o,o-EDDHA/Fe3+ >> DTPA/Fe3+ > EDTA/Fe3+ > HEDTA/Fe3+. The last three compounds cannot be used in calcareous soils, because the Fe-chelate is broken at pH values above 6, due to Zn, Mn or Ca substitution and the subsequent precipitation of Fe. For the most stable chelates, the stability sequence found in soils does not match the stability constant (log K°) sequence (Yunta et al., 2003b), which is o,o-EDDHA/Fe3+ (35.09) > EDDH4MA/Fe3+ (34.44) > EDDH5MA/Fe3+ (33.66) > EDDHSA/Fe3+ (32.79). This occurs because proton, Cu2+, Ca2+ and Mg2+ competition do affect equilibria differently. Therefore, when comparing the stability of different Fe-chelates, all possible reactions affecting them should be considered in the chemical speciation, instead of comparing only the log K° values.
o,o-EDDHA/Fe3+ and analogue molecules contain two diastereoisomers: meso and racemic. These diastereoisomers also behave differently. While racemic-o,o-EDDHA/Fe3+ presents a higher log K° (35.86) than that of meso-o,o-EDDHA/Fe3+ (34.15), for EDDH4MA the opposite occurs (35.54 for meso-EDDH4MA/Fe3+ and 33.75 for rac-EDDH4MA/Fe3+). Larger differences have been found among the hydroxyl-positional isomers of the chelates (Yunta et al., 2003c). For o,p-EDDHA/Fe3+ the log K° (28.72) is lower than that of o,o-EDDHA/Fe3+ (35.09). Moreover, p,p-EDDHA is not able to complex Fe in solution in a wide range of pH values. Whereas Fe is bound to 6 donor groups in o,o-EDDHA/Fe , in o,p- EDDHA/Fe3+ only five groups bind the Fe3+ ion (see Figure 5-5), allowing the sixth coordination position to be occupied by a water molecule. Other important difference is that o,p-EDDHA/Fe3+ may be easily protonated, and in fact the main species at pH values below 6.30 is the neutral species FeHo,p-EDDHA (Figure 5-5). Moreover, at pH values above 9.27 the main species is FeHo,p-EDDHA2-. All other o,o-EDDHA analogues form Fe-chelates with one negative charge, except EDDHSA and EDDCHA, which form Fe-chelates with three negative charges. It should be mentioned that the diagrams shown in Figure 5-4 have been drawn for a normal Cu availability in soils (limited to a maximum of 10 ^M of total Cu in soil solution), whereas Cu2+ largely affects the stability of o,p-EDDHA/Fe3+ (as well as those of other Fe-chelates at high pH values). In these conditions, o,p-EDDHA/Fe3+ is quite stable, but when the soil has more available Cu2+ this ion can displace more Fe3+ from the Fe-chelate.
Theoretical chemical speciation is a useful tool to predict the behaviour of Fe-chelates in agronomic conditions prior to their application. However, it should be considered that the main competitors are micronutrients such as Cu, whose availability in the soil is a quite low, and a small variation in the availability of that micronutrient may produce a different Fe-chelate speciation. Moreover, it is difficult to include sorption processes in the chemical speciation studies.
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