DIY fertilizer stock solution calculator: ppm per mL and solubility check

Why use a stock solution

Dry dosing a small tank means weighing fractions of a gram. Dissolving a larger amount in a bottle and dosing milliliters spreads the same weighing error over many doses. PPS-Pro is built that way, with a published 500 mL recipe ^[1], and EI’s write-up also describes dissolving the trace mix and dosing it as a liquid ^[4].

How the calculator works

You enter the final volume of the bottle (after topping up, not the water you started with), the grams of each salt, the dose in milliliters, the tank volume and how often you dose. For each dose it computes the grams of each salt delivered,

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

and then the mg/L of every nutrient with the same molar masses as the dry dosing calculator. Potassium and sulfate from different salts are added together. It also reports how many doses the bottle holds and how many weeks that lasts.

The solubility check

A salt can only dissolve up to a limit, and the limit drops in cold water. The PubChem records give grams per 100 mL of water, while the calculator measures grams per 100 mL of finished solution, so it converts each figure first. A saturated solution of S g per 100 mL of water is S ÷ (S + 100 × ρ) salt by mass, where ρ is the density of water at that temperature. Dissolved salt makes the solution denser than water, so taking the solution’s density as that of pure water gives a limit that is never higher than the real one. For Epsom salt the figure is for anhydrous MgSO4, so the limit is then scaled up by molar mass to grams of the heptahydrate you weigh out:

Salt	PubChem figure (g per 100 mL water)	Limit used (g per 100 mL of solution)	Source note
KNO3	35.7 at 25 °C	26.3	[5]
KH2PO4	22, no temperature stated (25 °C assumed)	18.0	[6]
K2SO4	12 at 25 °C	10.7	lowered by potassium chloride or ammonium sulfate [7]
MgSO4·7H2O	30 as anhydrous MgSO4, at 20 °C	47.2	23.1 g MgSO4 converted by molar mass [8]

These are single-salt figures. In a mixed bottle the salts share ions; PubChem notes, for example, that potassium sulfate’s solubility is decreased by other salts ^[7], and potassium nitrate is a potassium salt too. The calculator therefore warns above 80 % of a single-salt limit, and you should expect the true mixed-bottle limit to be lower.

Worked example: the PPS-Pro macro bottle

PPS-Pro’s macro recipe is 29 g K2SO4, 33 g KNO3, 3 g KH2PO4 and 20 g MgSO4 per 500 mL ^[1]. Per 100 mL that is 5.8 g K2SO4, 6.6 g KNO3, 0.6 g KH2PO4 and 4 g Epsom salt, all well under the single-salt limits. One milliliter in 40 L delivers 1.01 mg/L NO3, 0.10 mg/L PO4, 1.33 mg/L K and 0.10 mg/L Mg, matching the daily dose the author states ^[2]. A 500 mL bottle is 500 doses: about 71 weeks of daily dosing for a 40 L tank, or 36 weeks at the 2 mL a day an 80 L tank needs.

Practical notes from the method authors

• Keep macro salts and the trace mix in separate bottles; the PPS author advises against combining chemicals unless a recipe says so ^[2].
• Use soft water to make the solution; the PPS author suggests RO or distilled water when hard tap water leaves a sandy residue ^[3].
• Cloudiness or mold in a bottle is described by the PPS author as harmless and caused by impurities ^[3]; EI’s write-up stores the trace solution in a refrigerator against mold ^[4].
• Label every bottle with its contents, concentration and date, and keep it away from children and food.