Mixing dry-salt stock solutions: solubility, separation and storage

Why bottle your salts

Weighing 0.04 g of monopotassium phosphate for a small tank is below the resolution of many kitchen scales. Dissolving a larger, weighable amount into a bottle and dosing by the milliliter turns a small mass into a measurable volume. PPS-Pro is built entirely on this idea ^[1], and EI’s write-up describes dissolving the trace mix the same way: one tablespoon of CSM+B in 250 mL, where 20 mL equals a quarter teaspoon of dry powder ^[4].

The arithmetic

Make up to a final volume. The PPS-Pro instructions are explicit: add the salts, then “fill up both bottles with soft water to the 500 ml size” ^[1]. Every milliliter then holds grams ÷ final volume of each salt:

grams of salt per dose = grams in bottle × dose (mL) ÷ bottle (mL)

From there the dose in mg/L follows from molar masses, exactly as for dry dosing (ppm arithmetic). The stock-solution calculator does the whole chain and reports how many doses and weeks a bottle lasts.

Solubility: how much fits in the bottle

Each salt has a limit, given in its PubChem record:

Salt	Solubility in water	Notes
KNO3	35.7 g/100 mL at 25 °C	far more in hot water: 1 g in 0.5 mL boiling [5]
KH2PO4	22 g/100 mL	25 g/100 mL at 25 °C in another record entry [6]
K2SO4	12 g/100 mL at 25 °C	lowered by potassium chloride or ammonium sulfate [7]
MgSO4	30 g/100 mL at 20 °C	anhydrous basis [8]

Four practical consequences:

1. Potassium sulfate is the bottleneck. At 12 g per 100 mL it is the least soluble of the common macro salts, and PubChem notes that its solubility drops in the presence of potassium chloride ^[7]; other potassium salts in the same bottle, such as KNO3, share the potassium ion. The PPS-Pro macro recipe holds 5.8 g K2SO4 per 100 mL ^[1], about half the single-salt limit.
2. Cold storage lowers the limit. The figures are for room temperature. A bottle mixed warm, or near the limit, can crystallize in a cold room or refrigerator. The PPS author notes that warm water dissolves the recipe in minutes rather than overnight ^[1].
3. Margins matter in mixtures. The single-salt figures overstate what a mixed bottle can hold. The calculator warns above 80 % of any single-salt limit for that reason.
4. The figures are per 100 mL of water, not of finished solution. A bottle topped up to 100 mL holds less: the stock-solution calculator converts the limits on the safe side, for example to 26.3 g of KNO3 and about 47 g of Epsom salt per 100 mL of solution.

Keep phosphate away from iron and calcium

Phosphate forms practically insoluble compounds with iron and with calcium: PubChem describes both ferric phosphate and tricalcium phosphate as practically insoluble in water ^{[9, 10]}. In a concentrated bottle, mixing a phosphate source with iron or calcium salts risks a precipitate that takes both nutrients out of solution. That is the chemistry behind the two-bottle design used by PPS-Pro and EI: macros (including phosphate) in one bottle, iron and trace elements in another ^{[1, 4]}. The PPS author’s general advice is not to mix chemicals at all unless a published recipe calls for it ^[2]. EI Light doses its GH booster dry, once a week at the water change ^[4], which keeps calcium out of the phosphate bottle altogether.

Storage

• Water. The PPS author recommends soft water and suggests RO or distilled water if hard tap water leaves a sandy residue ^[3].
• Mold and cloudiness. Solutions can cloud, settle or grow mold; the PPS author attributes this to impurities and calls it harmless ^[3]. EI Light keeps its trace solution in a refrigerator to prevent mold ^[4].
• Light. The PPS author says bottles need not be kept in the dark but should be out of direct sunlight ^[3].
• Dry salts. Kept dry in sealed bags, the dry chemicals have no expiry date, according to the PPS author ^[2].

Safety

Fertilizer salts are chemicals, not food. Potassium nitrate in particular is classified as an oxidizer that may intensify fire (GHS H272), and is reported as a skin, eye and respiratory irritant in most notifications summarized by PubChem ^[5]. Store it away from heat and flammable material, label every bottle with its contents and concentration, keep bottles away from children and food, and follow the safety data sheet supplied with each product.