"Two bottles is just marketing to sell more product." In reality, two concentrates are a chemical necessity. Ca²⁺ and SO₄²⁻ stored together in concentrated form produce insoluble CaSO₄ - gypsum. Ca²⁺ and PO₄³⁻ produce Ca₃(PO₄)₂ - also insoluble. Mix all fertilizers into one concentrate and part of the elements have already precipitated before the solution ever reaches the plant. The A and B split is not marketing; it is chemical compatibility.
Quick glossary: Stock solution (concentrate) - a highly concentrated fertilizer solution diluted to working strength before use; typically 100-200× more concentrated than the working solution. Tank A - concentrate containing Ca(NO₃)₂ and usually micronutrients (Fe chelate, Mn, Zn, Cu, B, Mo). Tank B - concentrate containing KNO₃, KH₂PO₄, MgSO₄, and K₂SO₄; all components carrying sulfates and phosphates.
Why Ca and SO₄ Cannot Be in the Same Concentrate
The solubility of CaSO₄ (calcium sulfate, gypsum) in water is 2.4 g/L at 77°F (25°C). In the working solution, Ca at 150 mg/L and SO₄ at 200 mg/L are both well below the solubility limit - no problem.
But in a concentrate where everything is 100-200× higher, Ca and SO₄ concentrations exceed the solubility limit and CaSO₄ begins to crystallize. Even if the concentrate looks clear when mixed, CaSO₄ microcrystals have already formed and gradually settle - especially when cooled or stored for extended periods.
The same logic applies to Ca²⁺ and PO₄³⁻: the solubility product of Ca₃(PO₄)₂ is extremely low - these ions precipitate even at moderate concentrations. Phosphorus and calcium go in separate tanks, without exception.
What Goes in Which Tank and Why
Calcium, iron, and micronutrients go into Tank A because they stay stable in the acidic Ca(NO₃)₂ solution, while phosphates, sulfates, and potassium go into Tank B, since they would form insoluble compounds with calcium if stored together in a concentrate.
Tank A (calcium):
- Ca(NO₃)₂ - primary source of calcium and nitrate nitrogen
- Fe chelate (EDTA, DTPA, or EDDHA depending on pH)
- Micronutrients: Mn, Zn, Cu, B, Mo in chelated or soluble form
Why micronutrients go in Tank A: most micronutrients in an alkaline environment (as created by Tank B concentrate) and in the presence of SO₄ and PO₄ form insoluble compounds. In the acidic Ca(NO₃)₂ solution (pH 4-5 of the concentrate), micronutrients are more stable.
Tank B (phosphate-sulfate):
- KNO₃ - potassium and nitrate nitrogen
- KH₂PO₄ - potassium and phosphorus
- MgSO₄ - magnesium and sulfate
- K₂SO₄ - additional potassium when required
- (NH₄)₂SO₄ or NH₄NO₃ - if ammonium nitrogen is included in the recipe
How to Dilute Correctly - and the Typical Mistake
Dilution follows a set order: fill the reservoir with water to 50-70%, add and stir Tank A, then Tank B, top up to volume and adjust pH, because adding A and B together into a small volume of water forms a local calcium excess and micro-precipitate.
The dilution order is mandatory:
- Fill the reservoir to 50-70% with clean water
- Add Tank A - stir
- Add Tank B - stir
- Top up to final volume
- Adjust pH and verify EC
If A and B are added simultaneously or into a small volume of water, local Ca and SO₄ (or Ca and PO₄) concentrations will be momentarily high at the mixing point. The solution may clear within seconds - but microcrystals and micro-precipitate flakes have already formed.
Never add concentrate A into concentrate B or vice versa - not even a drop to "check" or "fix a mistake." Always dilute through water.
How to Mix A and B When the Tank Is Not Empty
Through water. The order above assumes a clean reservoir, and top-ups happen in a tank that already holds working solution and plants, where the same rule applies and is simply harder to follow.
Dilute each concentrate separately. Two pitchers with a few liters of tank water, the A dose into the first, the B dose into the second. Each pitcher is already at working strength, so no point exists where Ca meets SO₄ in concentrated form.
Pour with the pump running. First pitcher in, a minute of circulation, then the second. In a still tank the concentrate settles in a layer on the bottom and takes hours to blend.
Keep away from the intake. The pump will drag an undiluted stream straight into the line, and the precipitate forms inside the emitters, where nobody will see it.
A word on the ratio. Equal volumes of A and B is an assumption. The recipe sets the ratio, and in many of them it sits closer to 1:1.2 or 1:1.5. Pouring them evenly because it is convenient leaves the actual solution off the calculated one on potassium and magnesium, while EC still reads correct.
Three Tanks Instead of Two: When It Is Justified
Three tanks are justified for complex recipes with high Ca content or when NH₄⁺ needs to be dosed separately: Tank A holds Ca(NO₃)₂, Tank B holds KNO₃, KH₂PO₄, and MgSO₄, and Tank C holds NH₄NO₃ or acid for pH correction.
- Tank A: Ca(NO₃)₂
- Tank B: KNO₃, KH₂PO₄, MgSO₄
- Tank C: NH₄NO₃ or acid for pH correction
Three tanks offer more flexibility with automated dosing - each parameter is adjusted independently without affecting the others. For manual mixing, two tanks are sufficient for most recipes.
Three Mistakes That Cost the Most
Preparing one "convenient" concentrate by mixing A and B together. The result: some Ca has precipitated as CaSO₄ and Ca₃(PO₄)₂ on the bottom of the container or in the pump. The plant does not receive the calculated amounts of Ca and P. Symptoms appear 2-3 weeks later - and the grower looks for the problem in pH and the recipe, not the concentrate.
Adding Tank A into an empty or near-empty reservoir, then Tank B, without mixing in between. With a small water volume, the local Ca concentration is high - and precipitate can form when B is added even if it would not form in the final working solution.
Storing concentrates together where they can be confused. The mistake of "poured from the wrong container" with identical unlabelled bottles results in a double dose of A and zero B, or the reverse. Color-coding tanks A and B or labeling them clearly is basic operational hygiene.
How to Know Concentrates Are Prepared Correctly
- Both concentrates are clear with no sediment or cloudiness
- After diluting in the correct order, the working solution is clear
- EC matches the calculated value for the volumes of concentrate added
- For concentrates stored longer than two weeks: check for sediment on the bottom before use; if present, shake and check whether it dissolves with stirring - if not, mix a fresh batch
If you want to go deeper: Fertilizer Chemical Compatibility: What Precipitates and How to Avoid It is the full map of incompatible fertilizer pairs and the ion forms that produce precipitates in concentrated storage.