"Diluted with water — now everything can be mixed together." But some ion pairs form insoluble compounds regardless of dilution — the solubility product does not depend on how "carefully" you mixed. Ca²⁺ and SO₄²⁻, Ca²⁺ and PO₄³⁻, Mg²⁺ and PO₄³⁻, Ca²⁺ and silicate — these pairs always precipitate when the solubility product is exceeded, even if the precipitate is not immediately visible. An invisible precipitate is not an absent precipitate. A portion of the nutrients is already locked out before the solution reaches the root.
Quick Glossary
- Chemical compatibility — the ability of two or more substances to remain in solution without precipitating when mixed; determined by the solubility product of their ions
- Precipitate — an insoluble solid phase that forms when the solubility product of ions in solution is exceeded; may be visible (cloudiness, flakes) or microscopic and undetectable visually
- Solubility product (Ksp) — a constant characterising the maximum ion concentration in equilibrium with a precipitate; once exceeded, precipitation is inevitable regardless of how mixing is done
Key Incompatible Pairs in Hydroponic Fertilisers
Knowing these pairs is the foundation of correct concentrate preparation and dosing order:
Ca²⁺ + SO₄²⁻ → CaSO₄ (gypsum) — solubility 2.4 g/L. In a concentrate ×100–200 — precipitation is inevitable. In a working solution — depends on Ca and SO₄ concentrations. At Ca > 200 mg/L and SO₄ > 300 mg/L in the working solution — risk of micro-precipitation on standing.
Ca²⁺ + PO₄³⁻ → Ca₃(PO₄)₂ — Ksp is extremely small: 10⁻³² mol⁵/L⁵. These ions are incompatible at virtually any concentration above micromolar. Ca and P — always in separate concentrates, no exceptions.
Mg²⁺ + PO₄³⁻ → Mg₃(PO₄)₂ — less critical than Ca+P, but precipitate forms in concentrated solutions. Mg and P — also in separate tanks.
Ca²⁺ + silicate (SiO₃²⁻) → CaSiO₃ — calcium silicate is insoluble. Liquid silicate (typically potassium silicate K₂SiO₃) must never be mixed with calcium nitrate. Silicate requires a separate concentrate, or must be added last into a very dilute solution after Ca is fully diluted.
Fe³⁺ + PO₄³⁻ → FePO₄ — iron and phosphorus form poorly soluble compounds. Iron chelate is more stable than Fe²⁺/³⁺, but partial precipitation is possible in concentrated solutions or on prolonged storage. Micronutrients belong in tank A; phosphorus in tank B.
Ca²⁺ + CO₃²⁻/HCO₃⁻ → CaCO₃ — above pH 7.5–8.0 bicarbonates convert to carbonates and Ca precipitates. With alkaline source water that has not been pre-neutralised, the first cycle may deposit a CaCO₃ scale in the system.
Compatible Pairs That May Seem Problematic
Some combinations look "suspicious" but are actually compatible:
K⁺ and PO₄³⁻ — potassium and phosphorus. K₃PO₄ and KH₂PO₄ are soluble. This is why KH₂PO₄ can be placed in tank B alongside K₂SO₄ without issue.
NH₄⁺ and NO₃⁻ — ammonium and nitrate forms do not react with each other in solution at neutral pH. NH₄NO₃ is a standard fertiliser.
MgSO₄ and KNO₃ — compatible at any concentration. This is why they form the foundation of tank B.
Ca(NO₃)₂ and KNO₃ — compatible, but when prepared together in a concentrate they increase osmotic load and reduce stability at low temperature. Store separately if concentrate exceeds 30% by weight.
Silicate: Why a Separate Tank or Separate Dosing Order
Silicate (K₂SiO₃) is used in hydroponics to strengthen cell walls and provide partial pathogen protection. However, K₂SiO₃ has a pH of 11–12 in concentrated form and is incompatible with Ca, Mg, and micronutrients when stored together.
Two safe options:
- A separate tank C for silicate — diluted and dosed after A and B into heavily diluted water
- Add silicate to the reservoir first, then add water to dilute (silicate EC < 0.3), and only then add A and B
The rule: silicate and Ca must never be in the same stream before dilution.
Three Mistakes That Cost the Most
Mixing Ca(NO₃)₂ and K₂SiO₃ in concentrated form or in a small volume. CaSiO₃ precipitates instantly — a visible gel-like deposit in the canister or pipeline. Blocked nozzles, drippers, and pumps are guaranteed. Silicate must always be kept separate from Ca until full dilution.
Assuming that "micro-precipitation doesn't count." Micro-crystals of CaSO₄ or Ca₃(PO₄)₂ are invisible in a clear solution — but they deposit on the surfaces of pipes, nozzles, and check valves. Within a few weeks this becomes scale that reduces flow and provides a substrate for biofilm growth. The correct concentrate sequence and dilution order eliminates this systematically.
Preparing concentrate A at too low a room temperature. Ca(NO₃)₂ has reduced solubility below 10°C — a concentrated solution may crystallise during winter storage in an unheated space. Store concentrates above 15°C or reduce the concentration of tank A.
Signs That Compatibility Is Maintained
- Both concentrates are clear
- Pipes, nozzles, and pumps show no white or grey scale after a week of operation
- The working solution is clear immediately after mixing and remains clear after 24–48 hours of standing with no visible cloudiness or sediment
- EC matches the calculated value — if actual EC is lower than calculated at the same concentrate volumes, part of the fertiliser has precipitated and did not enter solution