How Big Should a Fertigation Stock Tank Be?
Tank volume follows stock draw, not nominal capacity. This guide calculates working volume, checks solubility and refill interval, and lists what a quote must document.
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A practical guide to working volume, stock concentration, refill intervals and tank selection
A stock tank is not sized by its nominal label alone. The useful volume depends on the injector ratio, irrigation flow, injection duration, target fertilizer dose, solubility limit, mixing method and how often an operator can refill it. A 500 L tank can be oversized for a small greenhouse zone and undersized for a field block; the number only becomes meaningful after those operating conditions are written down.
This article owns the tank-and-stock-solution decision. The buyer task is to specify a tank that can hold a stable, usable batch for the planned irrigation event without creating an unmanageable concentration, refill or cleaning problem.
Separate the tank roles before comparing litres
| Tank role | What it holds | Sizing question |
|---|---|---|
| Stock / mother-solution tank | Concentrated fertilizer solution that the injector draws into the irrigation line | How much stock will the injector consume during one planned event or refill interval? |
| A/B nutrient tanks | Separate concentrates whose combination would precipitate or become unstable | What volume and independent injection control does each formulation need? |
| Day or feeder tank | A mixed solution prepared for immediate use by a pump or injector | How much usable solution must be available while the operator is absent or the system is cycling? |
| Bulk storage / IBC | Undiluted liquid fertilizer or a transport container, not necessarily the final injector tank | Is the container compatible with the product and does it have a controlled transfer path? |
Suppliers often use "fertilizer tank," "mixing tank," "storage tank" and "IBC" interchangeably in titles. They are not interchangeable in a design. A tank with a large nominal capacity may have no agitator, no calibrated level scale, no drain, or no chemical-compatibility statement for the intended formulation. Treat the title as a discovery lead, not a specification.
The calculation starts with stock draw, not tank capacity
For a proportional injector expressed as 1:R, the first calculation is the stock-solution draw. If the irrigation flow is Q (in litres per hour), the nominal stock draw is:
- Stock draw (L/h) = Q irrigation (L/h) ÷ R
- Working volume (L) = stock draw (L/h) × injection time (h)
That is a planning calculation, not an approval result. Add an explicitly chosen operating reserve for refill timing, unusable heel, level-sensor margin and any mixing requirement. Do not hide that reserve inside a rounded-up tank size; record it as an assumption the buyer can change.
| Input | Example | What must be verified |
|---|---|---|
| Irrigation flow | 12 m³/h = 12,000 L/h | Measured at the operating zone, not a pump nameplate or a catalogue maximum |
| Injector ratio | 1:100 | Whether the supplier defines the ratio as stock-to-final solution and whether it stays stable at this flow and pressure |
| Injection duration | 2 h | Whether the crop program actually injects for two hours, including start-up and clean-water flush time |
| Calculated stock draw | 120 L/h | Stock depletion test on the installed system; ratio drift is possible if the mechanism or pressure changes |
| Calculated event volume | 240 L before reserve | Add the buyer's documented reserve and usable-volume limit; do not treat 240 L as a universal recommendation |
The example deliberately stops before choosing a 300 L or 500 L tank. The remaining decision is operational: how much reserve is needed, how low the injector can safely draw, and how much solution can be mixed without exceeding solubility or leaving a difficult heel. Fertilizer-calculation guidance also stresses that the final stock volume is the volume after fertilizer is added, not the volume of water poured into an already full container.
Working volume is different from nominal volume
| Volume term | Use it for | Buyer check |
|---|---|---|
| Nominal tank volume | Comparing the headline size of a vessel | Ask whether the quoted 500 L means geometric capacity, fill-to-overflow or a supplier marketing label |
| Usable working volume | The quantity that can be mixed and drawn while maintaining the required suction, agitation and level margin | Request the minimum and maximum operating levels, outlet elevation and any anti-vortex or foot-valve requirement |
| Reserve volume | Time to refill, operator absence, batch repeatability or emergency margin | State the reserve in litres or minutes; do not let the supplier assume it |
| Heel / residual volume | Solution that remains below the outlet or cannot be recovered without cleaning | Measure it with the actual tank, outlet and suction assembly; residual concentration may not represent the batch |
A conical or sloped bottom can improve drainability, but it does not prove complete recovery. An agitator can keep solids suspended, but it does not make an incompatible or insufficiently soluble fertilizer safe. These are separate engineering questions and must be documented separately.
Concentration is a second constraint
After calculating the stock volume, check the amount of fertilizer that must be dissolved in that volume. Stock concentration is not "how much fertilizer fits in the tank"; it is the nutrient or product mass required by the irrigation program divided by the usable stock volume. Compare the result with the fertilizer label or manufacturer's solubility data at the actual water temperature and quality.
- Use the target nutrient, not only product mass. A 20-10-20 product and a calcium nitrate product do not deliver the same nutrient mass per kilogram, so keep the agronomic target separate from the equipment calculation.
- Make the final volume correctly. Add the measured fertilizer quantity, then bring the solution to the intended final volume. Filling a tank with water first and adding a bag afterward changes the concentration and may overflow the vessel.
- Treat solubility as a hard limit. A tank can be large enough by volume and still be unusable if the required mass settles or crystallizes. The answer may be a lower stock factor, a larger tank, a shorter refill interval or a different fertilizer.
- Keep incompatible concentrates separate. If calcium, phosphate or sulfate combinations are not stable in the stock solution, use independent A/B tanks or an alternative mixing sequence. Do not solve a chemistry problem by increasing tank capacity.
Refill interval is a design variable
A tank that lasts one fertigation event is not automatically better than one that lasts a week. A longer interval reduces labour but increases the time a concentrated solution sits in the tank, and it may encourage settling, temperature change or formulation drift. A short batch can be easier to inspect and repeat, but it needs a reliable refill and mixing procedure.
| Operating pattern | When it can fit | Risks to control |
|---|---|---|
| One batch per irrigation event | Small zones, frequent operator access, formulation that should not sit for long | More refill labour; batch-to-batch weighing and final-volume errors |
| One tank for several events | Stable, fully soluble stock and a controlled level or recirculation routine | Concentration drift, evaporation, settling and an unnoticed empty-tank event |
| A/B or multi-tank dosing | Nutrient concentrates that should be separated until dilution | More pumps, level sensors, interlocks and independent cleaning records |
| Bulk IBC feeding a smaller day tank | Large sites that need fewer bulk deliveries but controlled daily preparation | Transfer compatibility, overflow protection, sediment, and the difference between bulk storage and injection duty |
Mixing hardware: ask what the tank actually does
| Feature | Why it matters | Evidence to request |
|---|---|---|
| Agitator or recirculation loop | Keeps a stable solution moving when the formulation requires it | Motor power, speed range, impeller type, wetted materials and whether the stated capacity is the agitated working volume |
| Bottom outlet and drainability | Reduces residual heel and makes cleaning observable | Outlet size, low-point geometry, valve material and a drawing showing the true low point |
| Level indication or sensor | Prevents dry running, overfilling and a silent change in ratio | Scale calibration, sensor type, switching points and integration with the injector interlock |
| Cover and light control | Limits debris and may reduce light-driven biological growth or degradation | Cover material, venting, access for inspection and any stated storage limit |
| Chemical-compatible body and seals | The tank can be exposed to concentrated fertilizer or cleaning chemicals | Exact resin or metal grade, gasket material, temperature range and a compatibility statement for the named product |
Do not transfer stainless-steel or HDPE claims from one supplier to another. A marketplace field such as "SS304" or "chemical resistant" is a starting point; the buyer still needs the exact wetted parts, seals, temperature, concentration and cleaning chemistry documented for the delivered variant.
Sourcing candidates for different tank roles
These offers are inquiry leads, not approved products. Listing fields are seller or platform statements. Confirm the usable working volume, the agitator duty, the outlet and drain geometry, the wetted materials and the delivered configuration before a tank is approved for production.
Acceptance test before the tank enters production
- Check the volume with water. Measure the level marks, usable draw volume, low-level heel and overflow path on the actual tank and outlet assembly.
- Run the injector at the real ratio. Use the installed zone flow, pressure, suction line and filter arrangement. Record stock depletion over a measured interval rather than trusting a dial or catalogue ratio.
- Test the formulation separately. Prepare the intended stock at the proposed concentration and water quality. Observe dissolution, settling, temperature change and residue for the full planned hold time.
- Verify the refill procedure. A second operator should be able to reproduce the final volume and fertilizer mass from the tank card without guessing what "full" means.
- Document the empty-tank response. Confirm the injector stops, alarms or switches to a safe state before the stock line draws air or a concentrated heel.
RFQ questions that expose a bad tank quote
- What are the nominal capacity, usable working volume, minimum draw level and residual heel for the exact quoted tank?
- Is the tank intended for storage, mixing, day-tank feeding or direct injection? Show the piping and outlet arrangement for the intended duty.
- Which materials contact the stock solution, including tank, outlet valve, pickup tube, impeller, shaft, seals and level sensor?
- What fertilizer concentration, temperature, pH range and cleaning chemicals has the quoted body and seal package been checked against?
- What agitator speed, power and mixing pattern apply at the buyer's viscosity and concentration? Is the stated capacity the agitated working volume?
- Can the supplier provide a drawing with dimensions, low-point drain, outlet elevation, overflow and access cover?
- What happens when the level falls below the pickup? Is there a low-level interlock compatible with the injector or pump?
- Can the supplier test the buyer's formulation for dissolution and settling, and provide the exact delivered variant and spare-part list?
Bottom line
Approve a fertigation stock tank only when the working volume covers the planned injection event, the concentration is inside the formulation's documented solubility and compatibility limits, the mixing and drain path are suitable, the refill method is repeatable, and the injector has a defined low-level response. A tank labelled 200 L, 500 L or 1000 L is a candidate size, not proof of usable volume, chemical compatibility, dose stability or production readiness.
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