**4. Conclusions, limitations and challenges for future research**

In the pot trial experiments conducted, it was found that the effectiveness of FeEDDHA components in delivering Fe to soil-grown plants is largely determined by their ability to remain in solution. The residence time of o,p-FeEDDHA in soil solution proved too short to significantly contribute to facilitating Fe uptake. The residence time of both racemic and meso o,o-FeEDDHA was much longer and both isomers did contribute to Fe uptake, approximately to the same extent on the time scale considered. o,o-FeEDDHA facilitated Fe uptake increased both the Fe content and the dry weight yield of the soybean plants. Contrary to racemic o,o-FeEDDHA, the residence time of meso o,o-FeEDDHA in soil solution was substantially compromised by plant-independent processes. Due to its longer residence time, racemic o,o-FeEDDHA is likely to remain effective for a longer time-span than meso o,o-FeEDDHA. The effectiveness of rest-FeEDDHA has not been separately assessed in the pot trials. In the study examining the effect of FeEDDHA treatment composition, it was concluded that o,o-FeEDDHA governed Fe uptake; the contribution of rest-FeEDDHA was marginal, at most.

The findings from the presented pot trial studies may serve appliers of FeEDDHA fertilizer to make a better selection out of the available products and help them to optimize the dosage and frequency of application. Furthermore, they may provide producers of

applied o,o-FeEDDHA. Apparently, as a result of Fe deficiency stress response mechanisms and development of the root system, the soybean plants had grown much more efficient with regard to Fe uptake. In only two weeks time, the soybean plants from the *racemic o,o 6* and *meso o,o 6* treatments took up an additional 0.36 mg of Fe per pot, which corresponds

Fig. 11. Fe uptake by soybean plants grown on Santomera soil for all FeEDDHA treatments. Error bars indicate standard deviations. Letters indicate the significantly different groups as identified by the Tukey post-hoc test including all FeEDDHA treatments and all moments of

In the pot trial experiments conducted, it was found that the effectiveness of FeEDDHA components in delivering Fe to soil-grown plants is largely determined by their ability to remain in solution. The residence time of o,p-FeEDDHA in soil solution proved too short to significantly contribute to facilitating Fe uptake. The residence time of both racemic and meso o,o-FeEDDHA was much longer and both isomers did contribute to Fe uptake, approximately to the same extent on the time scale considered. o,o-FeEDDHA facilitated Fe uptake increased both the Fe content and the dry weight yield of the soybean plants. Contrary to racemic o,o-FeEDDHA, the residence time of meso o,o-FeEDDHA in soil solution was substantially compromised by plant-independent processes. Due to its longer residence time, racemic o,o-FeEDDHA is likely to remain effective for a longer time-span than meso o,o-FeEDDHA. The effectiveness of rest-FeEDDHA has not been separately assessed in the pot trials. In the study examining the effect of FeEDDHA treatment composition, it was concluded that o,o-FeEDDHA governed Fe uptake; the contribution of

The findings from the presented pot trial studies may serve appliers of FeEDDHA fertilizer to make a better selection out of the available products and help them to optimize the dosage and frequency of application. Furthermore, they may provide producers of

**4. Conclusions, limitations and challenges for future research** 

with 50% of the total Fe uptake in the blank treatment.

application. (based on Schenkeveld et al., 2010b)

rest-FeEDDHA was marginal, at most.

FeEDDHA fertilizers with leads for optimizing the compositions of their formulations and for effectively marketing their products.

Although the processes examined in these pot trials also take place in a field situation, a translation of the results to a field situation should be treated with caution, because plant care and growth conditions differ strongly between the field and a conditioned greenhouse, not all processes affecting FeEDDHA concentration in the field have been considered, and the relative impact of the individual process may well be different in the field than in a controlled environment.

The presented studies persued insights on a level, transcending an individual soil or crop. Still, for practical reasons, only one plant species (soybean) and one soil (Santomera) have been used. This inevitably holds a risk of over-representation of soil-, species- or even cultivar specific peculiarities. Challenges for future research would therefore include carrying out comparative studies with different soils and crops, and conducting field trials to examine how the results from the pot trials relate to agricultural practice.

Another focal point for further research concerns the fate of FeEDDHA components in the soil-plant system. The results from the pot trial studies show that for most of the FeEDDHA components, the fate is determined by plant-independent processes. A better understanding of the soil processes affecting the effectiveness of FeEDDHA components, or in a more general sense, of Fe chelates applied as fertilizer, would enable a more efficient and soil specific application of Fe fertilizer products. Processes to examine more closely than reported so far would for instance include biodegradation, adsorption, cation competition and leaching.
