Why We Leave Silica Out of the Nutrient Reservoir

We don't use silica with CropSalt, in the reservoir or as a foliar, and our feed doesn't need it. We've watched silicates turn a clear, stable reservoir at pH 5.8–6.2 into one with pH swings, clouding, deposits and a film across the top. If the water, nutrients and equipment haven't changed and the problem shows up only when silica goes in, the silica is the cause. Stop adding it, clean the tank and remix.

Whether the concentrate looks clear in its bottle isn't the question. What matters is what it does after it's diluted into a nutrient reservoir and sits there between feeds.

The forms sold to growers

• Potassium silicate comes as a powder or liquid; AgSil 16H is a familiar powder. It supplies potassium as well as silicon and is strongly alkaline. Something that stays dissolved in an alkaline concentrate can behave very differently once it hits an acidic nutrient solution.
• Stabilized silicic acid is sold as monosilicic or orthosilicic acid; both names mean the same simple molecule roots take up. Stabilizers keep it from joining into larger molecules in the bottle. Potassium silicate also forms silicic acid after dilution, so that plant-available form isn't exclusive to the expensive liquids.
• Water-reactive silicon concentrates form silicic acid on contact with water. Some products marketed as monosilicic acid list ethyl silicates in their safety data, so the marketing name doesn't always describe what's in the bottle.
• Colloidal silica and mineral powders are dispersed particles or mineral solids. Their total silicon isn't the same as dissolved, available silicic acid. Sodium silicate is another alkaline source that adds sodium instead of potassium.

What happens at pH 5.8–6.2

At normal nutrient pH, dissolved silicon is mostly silicic acid. Those molecules can join into larger structures and eventually form suspended particles, gel or deposits. Concentration and the nutrient salts around them affect whether that happens, and calcium and magnesium matter when an alkaline silicate meets the feed.

Mixing order and local concentration matter too. A concentrated splash can create a high-pH reaction zone even when the tank reads 6.0, and bringing the pH down afterward doesn't reverse it.

A dilute enough silicic-acid solution can stay dissolved in this range, so pH alone doesn't prove fallout. But when the same CropSalt reservoir is stable without the additive and unstable with it, there's no reason to hunt for another variable.

Clouding is fallout

A persistent white haze from silica reactions is material coming out of solution. It doesn't have to settle. Fine particles stay suspended and make the solution look cloudy or milky; larger material settles or collects on the reservoir, pump, filters and lines.

The deposit can be silica or other minerals that dropped out because the additive changed the chemistry. Either way, they're no longer dissolved, and a normal EC reading doesn't prove the nutrient balance is intact.

We also see a film across the top and residue around the waterline. Silicates can form skins at exposed surfaces as they react with air and dry. Pinning down exactly what's in a film takes testing, but when it only forms with the additive, the connection is clear. Breaking it up with a pump doesn't dissolve it.

Why pH gets harder to manage

Potassium silicate adds alkalinity, so it takes more acid to reach target. Concentrated contact, incomplete mixing and ongoing reactions can keep the tank moving after the first adjustment. Products don't all shift pH equally, but the fix is the same when an additive starts the swings: leave it out.

Air stones and strong surface agitation release dissolved CO₂ and warm the water, which also moves pH. That doesn't explain away silica when silica was the only change. We use wavemakers for circulation and avoid unnecessary splashing and heat.

Available silicon and price

The lab attachments in a 2015 forum comparison covered a potassium silicate powder and four specialty liquids. Reported soluble silicon:

Those are lab results for those samples, not guarantees for every bottle today. They show that cheap potassium silicate shouldn't be written off as having no available silicon. They don't show what stayed dissolved in a complete nutrient reservoir days later.

For cost, checked October 25, 2026: a one-pound potassium silicate powder was listed at $19.80, and a specialty silicic-acid liquid's 100 mL bottle had a $61 regular price. At their published example rates:

Tax and shipping excluded. The powder listing was out of stock, and the liquid also showed a lower sale price. These are application-cost examples, not equal silicon doses or a CropSalt recommendation. The powder's 52.8% SiO₂ is about 24.7% total elemental silicon, a different measurement from the lab's soluble-silicon result. A true cost per gram of plant-available silicon would need matching current analyses, not old lab results applied to today's formulas.

What to do when silica causes a problem

1. Stop adding it.
2. If there's fallout or a film, empty the reservoir and remove the deposits from the tank and any serviceable equipment.
3. Rinse, then mix fresh CropSalt feed without silica.
4. Check that every outlet is flowing evenly again.

Don't keep adding acid to rescue a cloudy batch, and a bigger pump won't put the material back into solution.

A separate silica spray isn't needed either. When we foliar feed, we use the finished feed straight from the reservoir; see why we use our reservoir mix for foliar feeding. If you use a silica product anyway, keep it out of the reservoir and follow its foliar label; that rate doesn't come from our feed chart.