Solution Change Calculator

System type

Different systems accumulate salts at different rates.

Plant stage

Higher demand stages often need tighter intervals.

Reservoir volume (L)

Total working solution volume.

Daily water uptake (L/day)

Average top-off amount per day.

EC drift (mS/cm/day)

How quickly EC rises or falls on average.

Max acceptable EC drift (mS/cm)

Your tolerance before resetting the solution.

Average solution temperature (°C)

Warm solutions drift and degrade faster.

Allowable top-off replacement (%)

Change when top-offs replace this share of volume.

Example data table

System	Stage	Reservoir (L)	Uptake (L/day)	EC drift (mS/cm/day)	Max drift (mS/cm)	Temp (°C)	Allowable top-off (%)
Recirculating	Vegetative	80	5	0.12	0.8	22	60
Recirculating	Flowering	60	4	0.18	0.6	26	55
Drain-to-waste	Leafy greens	120	6.5	0.08	0.7	21	70
Ebb & flow	Seedling	40	2	0.05	0.5	20	65

Tip: Run these values in the calculator to see how drift, uptake, and temperature change your interval.

Formula used

EC drift interval

Days until your EC tolerance is exceeded

Interval_EC = Max_Drift ÷ Daily_EC_Drift

Volume replacement interval

Days until top-offs effectively replace part of the reservoir

Interval_Vol = (Reservoir_Volume × Allowable_% ) ÷ Daily_Uptake

Final recommended interval

Choose the stricter limit, then adjust for conditions

Base = min(Interval_EC, Interval_Vol)
Recommended = clamp(Base × Temp_Factor × Stage_Factor × System_Factor, 3, 21)

Factors are conservative multipliers that shorten the interval when conditions increase instability.

How to use this calculator

Measure your reservoir volume and average daily top-off amount.
Track EC at the same time daily to estimate drift per day.
Set a maximum drift you consider acceptable for your crop.
Enter your average solution temperature and plant stage.
Calculate, then follow the planned interval and next date.
Use the partial refresh tip when drift spikes between changes.
Export CSV or PDF to keep a simple maintenance record.

Why solution changes matter for crop stability

Nutrient solutions evolve as plants drink water and selectively absorb ions. Even with careful top-offs, the ratio of nitrogen, potassium, calcium, and micronutrients can drift away from your target. A scheduled change resets the profile, helping roots maintain predictable osmotic pressure and improving uptake consistency.

Using EC drift as an early warning signal

Electrical conductivity is a practical proxy for total dissolved salts. When daily EC drift is high, it usually indicates rapid concentration changes, evaporation, or imbalanced uptake. This calculator translates your measured drift into a time window, so you can choose a change interval before plants show tip burn, chlorosis, or slowed growth.

Volume replacement and real-world top-off behavior

Top-offs replace water, not the exact nutrient ratio that was removed. After enough top-offs, the reservoir is effectively “new water plus old salts,” which can cause accumulation of specific ions. The volume-based rule changes the solution when top-offs replace a chosen percentage of the reservoir volume.

Temperature, stage, and system effects

Warmer solution temperatures accelerate biological activity and can increase instability, so conservative schedules are recommended. Plant stage also matters: seedlings are sensitive, while heavy flowering and fruiting can pull nutrients unevenly. Drain-to-waste setups often require tighter resets because runoff and mixing patterns differ from recirculating systems.

Example data and practical interpretation

Example: a 80 L recirculating reservoir with 5.0 L/day uptake and 0.12 mS/cm/day EC drift, with 0.8 mS/cm maximum drift at 22°C, typically supports about one to two weeks between full changes. Compare with a 60 L flowering setup drifting 0.18 mS/cm/day, which generally shortens the interval. Use the exported CSV/PDF to log what worked and refine your drift estimate over time.

Scenario A: 80 L, 5.0 L/day, drift 0.12, max 0.8, 22°C, top-off 60%.
Scenario B: 60 L, 4.0 L/day, drift 0.18, max 0.6, 26°C, top-off 55%.
Scenario C: 120 L, 6.5 L/day, drift 0.08, max 0.7, 21°C, top-off 70%.

FAQs

1) What is a good default change interval if I have no measurements?

Start with 7–14 days for most recirculating gardens. Then measure EC drift for several days and adjust the schedule using this calculator for your specific reservoir and temperature.

2) How do I estimate daily EC drift accurately?

Measure EC at the same time each day after mixing. Record the change per day for at least 3–5 days. Use the average drift value as the calculator input.

3) Why can volume replacement be the limiting factor?

Top-offs mainly replace water. Over time, certain ions can accumulate while others deplete. Changing the solution after a chosen replacement percentage helps reset nutrient ratios and prevents salt imbalance.

4) Does pH affect when I should change the solution?

Yes. Frequent pH swings, heavy buffering, or rapid pH drift can indicate nutrient imbalance or microbial activity. If pH is unstable, use a shorter interval even if EC looks steady.

5) Should I always do a full change, or can I do partial refreshes?

Partial refreshes can help between full changes, especially during warm periods or high drift. However, a full change is still needed periodically to fully reset the ion profile.

6) How does temperature change the recommendation?

Higher temperatures can increase drift and biological activity, reducing stability. The calculator applies a conservative temperature factor to shorten the interval when average solution temperature is elevated.

7) Can I use this calculator for soil or coco runoff mixing?

Yes, as a planning tool. Treat your “reservoir” as the volume you mix at once. Use measured drift and your refill pattern to estimate when a fresh batch will be beneficial.