Nutrient Accumulation Calculator
The fertiliser calculator tells you what goes in this week. This one answers what comes next: if you keep dosing, where does the water settle?
Pick the nutrient to track
Nitrate, phosphate or potassium — each is calculated separately. Starting from a preset is easiest.
Enter the weekly dose
The weekly mg/L figure from the fertiliser dosing calculator. With a commercial liquid, convert the label figure for your volume.
Enter the water-change fraction
The percentage you change once a week. This single number is what decides where the balance settles.
Account for your tap water
If the mains water carries nitrate, water changes cannot bring you below that level. The cleaner the change water, the lower the steady state.
Read the table
The level rises over the first weeks, then flattens. The 'time to approach steady state' row shows how long that takes.
Why does EI call for 50% weekly water changes?
EI deliberately overdoses so no nutrient is ever limiting. The weekly water change is what stops that surplus from stacking up. The maths is simple: at 50%, the peak always settles at exactly twice the weekly dose. Drop to 25% and the peak rises to four times the dose.
How high does nitrate go with no water changes?
There is no ceiling. Without water changes there is no steady state at all; the level climbs linearly by (dose − plant uptake) every week. It stops only if uptake happens to equal the dose, which is hard to hit and breaks with every change to the layout.
How does nitrate in tap water change the result?
It adds directly to the steady state. With 10 mg/L in the mains, the tank never goes below 10 mg/L — the water change brings that same water in. Where mains levels are high, either blend with RO water or lower the dose accordingly.
Why does my measured level not match the calculation?
Three common reasons: plant uptake differs from what you entered, the substrate is binding and releasing nutrients, or the test kit is off. Drop tests can be ±20% out on nitrate in particular. The model assumes no source other than dose, uptake and water changes; tuning the uptake figure to match your readings is the practical fix.
Does the same calculation work for phosphate and potassium?
Yes. The cycle depends on the dose–uptake–water-change pattern, not on the nutrient's chemistry, so the formula is unchanged. Only the numbers differ: EI runs phosphate around 3 mg/L weekly against nitrate at 20 mg/L.
How long does it take to reach steady state?
It depends on the water-change fraction. At 50% weekly you reach 95% of the steady state in five weeks; at 25% it takes eleven. In a newly set-up tank, low early readings are normal — the system has not settled yet.