In an unchelated nutrient solution, iron, zinc, manganese, and copper are highly reactive. As pH drifts above 6.0 — which happens routinely in recirculating reservoirs as plants take up nitrate — these metals form insoluble hydroxides and phosphates that drop out of solution. Once precipitated, they're effectively invisible to the root system. Plants will show the classic interveinal chlorosis even though the metals are still physically present in the reservoir.
EDTA chelation works by wrapping a synthetic ligand around the metal ion, forming a stable ring structure that prevents it from reacting with hydroxide, phosphate, or carbonate. The chelated metal remains dissolved across the 5.5–6.5 reservoir pH range and stays available for root uptake. Boron and molybdenum behave differently — their ionic forms (borate and molybdate) are naturally stable across this pH range, so chelation isn't required.
The practical implication: a Micro Green stock solution stored in a dark container will hold its full micronutrient availability for six months or more. UV and visible light can break the chelate ring through photodegradation, which is why the bottle goes in a cabinet, not on a windowsill.
For deeper coverage of the seven trace elements and what each one does, see Essential Micronutrients for Healthier Plants.