No nutrient needed for most of the cycle
A microgreen lives off the reserves in its own seed for almost its entire life. It does not need a nutrient solution because it has no time to become deficient.
Plain water is sufficient for most varieties. Some growers add a very weak solution — EC 0.4–0.8 — from day five, but it is not necessary and can encourage mould.
This makes microgreens the cheapest thing to grow here: no nutrients, no pH adjustment, and a harvest in ten days.
Density, light and airflow
Sow thickly. A microgreen crop is a carpet, not a row — scatter seed densely across the surface and press it down so it contacts the medium.
Germinate in darkness or low light for the first two to four days; this encourages the stems to elongate and lift the seed hulls clear. Then move to bright light — not direct sun — for the remaining week. Light at the right stage is what makes them green and thick rather than pale and leggy.
Airflow is the critical factor. Dense sowing with still air produces mould within days. A small fan moving air across the tray is the difference between success and a fuzzy green mat you have to throw away.
The mould problem
Damping off and mould are the defining failure of microgreens, and they come from the same three causes: too much water, too little airflow, and seed sown too densely in a warm still room.
Water from below if you can — a tray of water the pot sits in — so the surface stays drier. Never let water sit on the leaves. Keep a fan running. If mould appears, remove the affected area immediately and increase airflow before it spreads.
Bottom-watering and airflow solve almost every microgreen problem.
Harvesting
Harvest when the first true leaves appear — the second pair, after the seed leaves. Cut with scissors just above the medium, in one clean pass.
Do not wash after cutting; microgreens are delicate and wet leaves spoil within hours. Shake off any loose seed hulls and eat or refrigerate immediately.
They do not keep well — two or three days in a sealed container in the fridge at most. Grow small quantities often rather than a large batch you cannot eat.