The behaviour of EDDHA isomers in soils as influenced by soil properties.

FeEDDHA products are applied to correct iron chlorosis in plants and consist of a mixture of EDDHA isomers chelated to iron. In this study such mixtures have been divided into four (groups of) isomers: racemic o,o-EDDHA, meso o,o-EDDHA, o,p-EDDHA and rest-EDDHA. The physical and chemical properties of these isomers differ and hence does their ability to deliver Fe to plants. To come to a soil-specific iron fertilization recommendation, the behaviour of the EDDHA isomers in the soil needs to be understood. This behaviour has been examined in a soil interaction experiment as a function of time, and it has been related to soil properties. The isomer fractions remaining in solution can be ranked racemic o,o-FeEDDHA > meso o,o-FeEDDHA > rest-FeEDDHA > o,p-FeEDDHA, regardless of soil properties. The o,o-EDDHA isomers largely determine the Fe concentration in solution. Although rest-EDDHA also consists of compounds that chelate Fe more strongly than meso o,o-EDDHA, the latter is on average better capable of keeping Fe in solution upon interaction with soil. The principal adsorption surface differs per EDDHA isomer. For racemic o,o-FeEDDHA it is organic matter, for meso o,o-FeEDDHA it is iron (hydr)oxide and for o,p-FeEDDHA clay minerals. Cu and Al are important competing cations. Cu forms soluble complexes with o,p-EDDHA, and Al with meso o,o-EDDHA not chelated to Fe. Al is likely to affect the effectiveness of a potential shuttle effect. The tendency of o,p-FeEDDHA and rest-FeEDDHA to be removed from solution, makes these isomers less effective as iron fertilizer in soil application, in particular on clay soils.