A white crust on the substrate surface looks the same in four different cases, and what you should do differs in each. Before acting, name the crust. Three properties separate them: whether it dissolves in water, whether it smells, and what it looks like under a loupe.
Quick glossary: Salt crust - a layer of mineral salts left on the substrate surface after water evaporates. Drain EC - the electrical conductivity of the solution leaving the substrate; it reports salt concentration around the root rather than in the tank. Matric potential - a measure of how tightly the substrate holds water; the drier it is, the lower the value in hectopascals. Flushing - applying clean water in a volume large enough to carry accumulated salts out.
A White Crust Is Four Different Things
Fertilizer salt dissolves. A drop of water removes it without a trace, there is no smell, and under a loupe it is crystals with no threads. This is the most common case in substrate culture with feeding.
Carbonate deposit from hard water is crystalline too, but it goes into water reluctantly and holds as a film along the tray edges and rims, where water stood longest. Crust from fertilizer and crust from tap water often sit together.
Mold does not dissolve, and it has structure: chaotic threads climbing the stem, the rim and the lid, plus a smell of its own. Root hairs are white as well. But they come off every seed as an even collar in the narrow band where the stem enters the substrate. The catalog separates that pair in a dedicated line on 17 cards out of 70, and microgreen diseases covers it in more detail.
A green or grayish tint takes the crust out of this list entirely: those are algae, which need light and standing moisture at the surface.
Why Salts Gather in the Top Layer
The direction water moves decides everything. Under bottom watering water rises by capillary action, evaporates at the surface and leaves behind everything it carried. The top layer works as the evaporator, and it is the layer that takes up the salt.
Montesano and colleagues (Frontiers in Plant Science, 2015) cut the substrate profile into three equal layers after a subirrigated tomato cycle. With the solution supplied at 2.0 dS/m, extract EC was 2.6-3.2 dS/m in the bottom layer, 3.1-3.6 in the middle and 4.6-5.4 at the top. The gradient across the block came out roughly twofold.
One detail from that work matters for irrigation settings. The drier substrate, at -60 hPa matric potential against -30, produced a smaller gradient, because there was less water to carry salt upward. A permanently wet top layer accumulates faster than one that gets a chance to dry.
For microgreens this is not theory. Bottom watering appears on 61 of the 70 catalog cards, and on 18 it is named the only acceptable method. So the whole catalog runs on exactly the regime that drives salts into the top centimeter, which is where the seed sits.
Hard Water Leaves a Crust With No Fertilizer at All
A microgreen tray is usually watered with plain water, and the crust still appears. The water itself is the source. At GH 12°dH, typical for many regions of Ukraine, it already carries around 120 mg/L of calcium and 20-30 mg/L of magnesium before any fertilizer.
Those cations do not go anywhere. Water evaporates, calcium and magnesium stay at the surface, and by day ten of the cycle the rims show what twenty waterings brought in. Water hardness is the only variable here, and a drop test measures it in five minutes.
The substrate makes its own contribution. Fresh coco releases sodium and potassium, and unprocessed material adds its own exchange on top, so the first days on unbuffered coco give elevated EC with no watering mistake behind it.
What the Crust Costs, and When It Means Nothing Yet
The crust itself does no damage. It marks the fact that EC in the top layer runs above what you supplied, and that is what affects germination and the root. So the order is fixed: measure first, decide second.
What the measurement says. Drain EC running 0.3-0.5 above the supplied solution is the working norm; running 1.0-1.5 or more above it means accumulation. Both numbers and the sampling method live in the article on drain EC, and without them the crust stays a spot in a photo.
What the trials say. Islam and colleagues (International Journal of Biological Macromolecules, 2019) grew wheat microgreens for eight days on organic media with sodium from 0 to 100 mM: plant sodium content rose with concentration, while a moderate 12.5-25 mM raised phenolics, flavonoids and anthocyanins. Zeng and colleagues (Foods, 2022) name the other side directly, that applying NaCl to microgreens carries a risk of reduced productivity. Eight days is enough for salt in the top layer to become an active factor.
Three Mistakes That Cost the Most
Scraping the crust off and carrying on. The deposit goes in ten seconds and the cause stays, because the crust is the part of the salt load that already reached the surface. Removal comes after the drain measurement, once you know how much to flush.
Flushing just in case. Flushing solves one situation out of three. On fresh coco elevated drain EC comes from substrate exchange, and under plant stress it rises because uptake dropped; in both cases a flush spends water and time for nothing.
Taking mold for salt. This one has the highest price, because a day of delay on a tray with a mold focus costs the tray. The check takes a second: a drop of water dissolves salt and does nothing to threads.
How to Know Salinity Is Under Control
The gap between supplied EC and drain EC stays within 0.3-0.5 and does not grow week over week. The substrate surface dries between waterings, with no permanently wet top layer.
Irrigation water hardness is measured, and above 12°dH you have a decision in hand: dilution with RO or a recalculated recipe. Any white deposit that appears, you named before you did anything to it. Rims and lid are clean: no threads, no green.
If you want to go deeper: osmotic stress shows what happens to the root at elevated concentration, once the crust is no longer the only symptom.