Iron Humate
Iron humate is iron bound to humic acid. Because humic acid carries many carboxyl and phenolic groups, it can bind iron ions and keep part of them in a form that plants can take up. Iron humate products are used in fertilizers to supply iron, alongside synthetic chelates such as EDDHA-Fe.
How humic acid binds iron
Iron is held mainly by the carboxyl (–COOH) and phenolic (–OH) groups of humic acid. Two things decide how much iron is bound and whether it stays in solution: the pH and the chemistry of the humic acid itself, such as its degree of humification and its content of acidic groups.
What research shows about pH
A study of iron(II) and several soil humic acids (Boguta et al., 2019) found:
- About twice as much iron was bound at pH 7 as at pH 5 (on average about 117 vs 57 cmol/kg).
- At pH 7, binding increased with the content of carboxyl and phenolic groups and with the degree of humification.
- At pH 5 iron was held mainly as a chelate; at pH 7 bridging between humic molecules became more important.
- The amount of iron bound was far below the number of acidic groups, because neighbouring groups and folding of the humic molecules limit access.
- At pH 5 the complexes stayed soluble over a range of iron concentrations; at pH 7, iron hydroxides formed and the humic–iron complexes tended to precipitate together with them.
What this means in practice
- Humic acids with more acidic groups and a higher degree of humification bind more iron.
- In neutral and alkaline conditions, iron humate solutions can lose stability as iron hydroxides form; test liquid blends at the pH you will use.
- In calcareous, high-pH soils, synthetic chelates such as EDDHA-Fe are usually used where strong iron deficiency must be corrected; iron humate is used more as part of humic-based fertilizer formulations.
For the general chemistry of humic acid, see humic acid vs fulvic acid and where humic acid comes from.
Frequently asked questions
What is iron humate?
Iron bound to humic acid, used in fertilizers as a source of iron.
Does pH affect how much iron humic acid binds?
Yes. In the study cited here, humic acids bound about twice as much iron at pH 7 as at pH 5, but the complexes were less stable in solution at pH 7 because iron hydroxides formed.
Is iron humate the same as chelated iron?
Both bind iron to an organic molecule. Synthetic chelates such as EDDHA-Fe hold iron more strongly at high pH; iron humate is a natural humic-based form.
What groups in humic acid bind iron?
Mainly the carboxyl and phenolic groups.
Reference
- Boguta, P., D’Orazio, V., Senesi, N., Sokołowska, Z. and Szewczuk-Karpisz, K. (2019). Insight into the interaction mechanism of iron ions with soil humic acids: the effect of the pH and chemical properties of humic acids. Journal of Environmental Management 245: 367–374. doi:10.1016/j.jenvman.2019.05.098
