In what order should I mix nutrients?

Q: In what order should I mix nutrients?

Mix nutrients into the full volume of water in this order: silica first, then Cal-Mag or part A, then part B, then any additives, stirring between each, and adjust pH last. Silica first because it is strongly alkaline (pH about 11-12). Calcium and the sulfates and phosphates in part B must meet only once diluted, or they precipitate. Then check EC against your target.

Short answer

Into the full volume of water: silica first, then Cal-Mag or part A, then part B, then additives, stirring between each, and pH last. Then check EC. The order stops calcium meeting sulfate and phosphate at high concentration, and keeps alkaline silica from shocking a mixed feed. Established chemistry.

Why

Calcium forms insoluble calcium sulfate (gypsum) and calcium phosphate when it meets sulfate or phosphate concentrated. At feed strength they stay dissolved; at concentrate strength they don't ^[1]. That is why feeds come as A and B, and why each goes into the water separately.

Classic nutrient solutions have always been made this way: separate stock solutions, each diluted into the tank in turn ^[2].

Silicon is a beneficial element, not an essential one ^[3]. Potassium silicate stock is strongly alkaline, around pH 11–12. Added first to plain water it disperses before it can spike pH around calcium and iron.

pH comes last because every salt, and silica most of all, shifts it. Adjust before the salts and you adjust twice.

The numbers

Step	What	Wait
0	Water, measure its EC; 18–22 °C (64–72 °F)	n/a
1	Silica (potassium silicate), 20–50 mg/L Si if used	Stir, 2–5 min
2	Cal-Mag, if needed	Stir
3	Part A (calcium, iron)	Stir
4	Part B (phosphate, sulfate, micros)	Stir
5	Additives	Stir
6	pH adjust (hydro: aerate 30–60 min first)	Re-read
7	Check EC	n/a

What to do

Fill with the full volume of water first.
Add each product separately, stirring between.
Use a separate syringe or cup for each bottle.
Adjust pH in small doses once everything is in.
Confirm EC against your stage target.

Safety: pH adjusters and silicate are caustic or acidic. Gloves and eye protection.

Common mistakes

A and B in the same measuring cup.
Silica added last, into an acidic mix.
pH adjusted before the nutrients.
Mixing into a little water, then topping up.

Full article: Hydroponic salts explained
Articles: silicon
Answers: pH before or after nutrients · storing mixed feed
Terms: A-B nutrient · stock solution

Silica is silicon dioxide (SiO2); growers use the word loosely for any silicon supplement added to stiffen stems and improve stress tolerance.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Sulfate (SO4 2-) is the form in which plants take up sulfur, delivered mainly by magnesium sulfate and potassium sulfate in mineral feeds.Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.An A/B nutrient is a mineral feed split into two concentrates so that calcium (part A) is kept apart from sulfate and phosphate (part B), which would precipitate together.A stock solution is a concentrated nutrient mix, often 100x or 200x final strength, diluted into the reservoir; calcium and sulfate/phosphate stocks must stay separate.Silicon (Si) is a beneficial element, not strictly essential, that plants absorb as silicic acid and deposit in cell walls, stiffening tissue and aiding stress tolerance.A beneficial element is one that improves growth or stress tolerance in some plants without being strictly essential, such as silicon, sodium, cobalt and selenium.Potassium silicate is a strongly alkaline liquid or powder used to add silicon (and some potassium) to feeds; it goes into the water first, before other nutrients.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.

Follow-up questions

Why do I add silica first when mixing nutrients?

Silica goes in first, into plain water, because potassium silicate is strongly alkaline, around pH 11-12. Added to a mixed, acidic nutrient solution it creates local pH spikes that can precipitate calcium and iron, while silicic acid polymerises at low pH. Stir it in, wait a few minutes, add the A and B nutrients, then adjust pH last.

Does part A or part B go in first?

Add part A first, stir, then part B, each into the full volume of water. Part A carries calcium and iron; part B carries the sulfates and phosphates. At feed strength they stay dissolved, but if they meet concentrated, calcium sulfate and calcium phosphate precipitate. Never pour A and B into the same measuring cup.

When should I add pH down?

Add pH down or pH up last, after every nutrient and additive is in and stirred, because the salts themselves change pH. In hydro, aerate a fresh mix for 30-60 minutes before the final adjustment, since carbon dioxide loss raises pH in the first hours. Add adjuster in small doses and re-read after mixing.

Sources

Sonneveld C, Voogt W, 2009. Plant Nutrition of Greenhouse Crops. Springer. doi.org/10.1007/978-90-481-2532-6

Hoagland DR, Arnon DI, 1950. The water-culture method for growing plants without soil. California Agricultural Experiment Station Circular 347. archive.org/details/watercultureme3450hoag/

Epstein E, 1994. The anomaly of silicon in plant biology. PNAS 91(1):11-17. doi.org/10.1073/pnas.91.1.11