Iron and trace elements: what chelates do and how to dose them

Iron and the other micronutrients

Plants need eight micronutrients in small amounts: boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc ^[4]. Iron is the one aquarists dose and test most. It plays a central role in respiration and photosynthesis, and a shortage reduces chlorophyll production ^[1]. Plants absorb it as ferrous (Fe²⁺) or ferric (Fe³⁺) iron ^[4].

What iron deficiency looks like

Iron does not move from old leaves to new ones, so a shortage shows first in young growth: yellowing between the veins, with a sharp line between the green veins and the yellow tissue, progressing to whitish-yellow leaves in severe cases ^[1]. Manganese deficiency looks similar on young leaves but with a diffuse, not sharp, boundary ^[1]. Magnesium is the common confusion: it also causes interveinal yellowing, but because magnesium is mobile the symptoms start on older leaves ^[1]. Tropica’s article on Pogostemon helferi describes iron deficiency in that plant the same way, as yellowish new leaves, corrected with a micronutrient fertilizer ^[12].

Why iron comes chelated

Dissolved iron is scarce in neutral and alkaline water. Chelation is the fix: an organic molecule (the chelate, from the Greek for “claw”) binds the metal ion into a soluble complex. Montana State University’s micronutrient module notes that chelation can raise the soluble iron concentration more than a hundred-fold, without which there would be too little iron available at pH levels near 8 ^[2]. Natural chelates such as humic and fulvic acids do this in soils; fertilizers use synthetic chelating agents, including EDTA, HEEDTA, NTA, DTPA and EDDHA ^[3]. The same module notes that chelated micronutrients cost more but can be considerably more effective per unit of nutrient ^[3].

The chelating agents differ in how firmly they hold iron as pH rises. The extension sources cited here list the agents but do not publish a stability range for each, so this page does not quote one. What manufacturers publish is the form and content of each product.

What iron products declare

Product	Iron	Chelate	Notes
Plant-Prod Chelated Iron 13.2%	13.2 % Fe	EDTA (68 %)	[5]
Plant-Prod Chelated Iron EDDHA 6%	6 % Fe	EDDHA (38.7 %)	sold for soil application [6]
Plant-Prod Chelated Micronutrient Mix	7 % Fe, plus Mn, Zn, Cu, B, Mo	EDTA (42 %) + DTPA (14 %)	[7]
Seachem Flourish Iron	1 % Fe	ferrous gluconate	liquid [9]

Montana State’s micronutrient table gives iron EDTA fertilizers a typical range of 5–14 % Fe ^[2]. To turn any of these percentages into a dose, use grams × 1000 × (percent ÷ 100) ÷ liters; the dosing calculator includes the Plant-Prod products as presets.

How much iron?

The dosing methods aim for small numbers, and they do not agree:

• Seachem’s directions dose Flourish Iron to maintain about 0.10 mg/L of iron ^[9].
• EI targets 0.2–0.5 mg/L “or higher” ^[10].
• PPS-Pro adds about 0.05 mg/L a day through its trace bottle ^[11].

Seachem adds a practical caution: because iron is used up quickly, test within 30 minutes of dosing ^[9]. A low reading hours after dosing does not prove the dose was too small.

Choosing a trace source

The PPS author lists several acceptable trace sources, including micronutrient mixes chelated with EDTA plus DTPA and EDTA-only CSM+B, and says any good-quality trace product can substitute ^[11]. When comparing products, look at three things on the label: the iron percentage, the chelating agent, and which other micronutrients are present. For example, the base Plantex CSM lists no boron ^[8], while the Plant-Prod micronutrient mix lists 1.3 % ^[7].

Keep iron and trace solutions separate from phosphate-containing stock bottles; the stock-solution guide explains why.