Fertigation Tank Capacity: Size Stock Storage Around Injection and Refill Cycles

A fertigation tank should hold the stock solution your fertigation system will consume before the next practical refill, plus a documented operating reserve and the liquid the suction system cannot safely withdraw. Divide that total by the permitted fill fraction to obtain the minimum nominal capacity. Then check that the selected tank can be mixed, drained, accessed and monitored, and that the injector can deliver the required stock within the available irrigation window.

This guide sizes a nonpressurized stock tank feeding a dosing device. A pressure or bypass fertilizer tank works differently: irrigation water enters the vessel, dilutes its contents and carries fertilizer out. Its pressure rating and changing concentration require a different design assessment. The storage calculation below must not be used to specify a pressure vessel. For that separate operating mechanism, see why concentration changes in a bypass fertigation tank.

1. Define what each litre represents

Catalog capacity is a geometric or nominal figure. It is not automatically the amount available to the crop. Establish the following boundaries before comparing quotations; otherwise two apparently similar tanks may offer different working storage.

Measure levels in litres using a verified level-to-volume relationship. A curved base can make a centimetre near the bottom represent less liquid than a centimetre higher up. Record whether the starting and ending measurements include the suction line. Stock charged into a line, captured during maintenance or left as a heel is not automatically fertilizer delivered to plants.

2. Build demand from injection events

For an event with a stable, measured stock draw rate, calculate V_event [L] = q_stock [L/h] × t_injection [h]. Add event volumes to obtain D [L] = Σ V_event over the actual interval between refills. With variable rates, use metered totals or sum shorter time steps; a single nameplate pump rate is insufficient evidence.

The storage condition is N ≥ (D + R + H) / f, where every volume is in litres and f is a decimal between zero and one. This assumes the prepared stock remains suitable throughout the interval and the whole permitted draw can be mixed and withdrawn consistently. Add any separate, uncounted withdrawal to D before applying the equation. Do not count the same line-fill volume twice.

If only irrigation flow and an injection percentage are initially available, write down the percentage basis. In the example below, stock percentage means stock volume divided by carrier-water volume before injection: q_stock = Q_water × 1,000 × r. A device that defines its ratio against final mixture volume requires the appropriate conversion. Confirm the convention and calibrate actual draw under operating pressure; the nutrient-target-to-injection calculation explains the preceding dose calculation.

3. Work through a three-zone storage example

The following figures are an original, hypothetical planning exercise, not a crop recipe or a recommendation for injection percentage. The listed hours are the active injection periods; stabilization and clear-water displacement occur outside them. Assume the three zones draw the same compatible stock, sequentially, once per day.

Total daily withdrawal is 240 + 135 + 168 = 543 L/day. For this exercise, assume the selected 1.5-day refill window contains 1.5 equivalent days of demand: D = 543 × 1.5 = 814.5 L. That multiplier is valid only for this assumed schedule. If discrete zone events cluster before the next staffed refill, add the events in the busiest actual window instead of multiplying a daily average.

Choose a project-specific reserve of 10% of interval demand, so R = 81.45 L. With a measured 40 L heel and an approved 90% fill limit, N ≥ (814.5 + 81.45 + 40) / 0.90 = 1,039.94 L, approximately 1,040 L nominal minimum. Neither 10% nor 90% is a universal design requirement.

A nominal 1,000 L tank provides only 0.90 × 1,000 − 40 = 860 L of usable draw, less than the 895.95 L demand-plus-reserve requirement. It fails this stated case. A 1,200 L candidate provides 1,040 L of usable draw, leaving 144.05 L above that requirement. Here “1,040 L nominal minimum” and “1,040 L usable draw” happen to share a number but describe different quantities. Verify the candidate's actual geometry and mixing limits before selecting it.

Concentration is a separate calculation

If the hypothetical prepared stock contains 100 g of fertilizer product per litre of final solution, the 543 L daily draw contains 54.3 kg of product. That is not 54.3 kg of a particular nutrient. The nutrient mass depends on the product analysis. This concentration is illustrative only: confirm product solubility at the lowest expected solution temperature, source-water chemistry and permitted mixing practice. For liquid concentrates, distinguish product litres, density and product mass. Prepare to the verified final solution volume; adding solids to a stated water volume can change the final volume.

4. Check the injection window before increasing storage

The delivery check is required stock rate = event stock volume / permitted injection time. If Zone A still requires 240 L but only 1.25 h remains for injection, the required rate becomes 192 L/h. The stock volume is unchanged, but its delivery rate and instantaneous dilution change. An agronomist must confirm that the revised application pattern remains suitable, and the dosing device must be verified at that flow and pressure.

A larger tank cannot compensate for an injector that delivers too slowly. Reserve measured time to pressurize and stabilize the zone, deliver stock, then displace nutrient solution to the intended outlets with clear water. Do not shorten the final displacement merely to empty the stock tank. Agriculture Victoria's fertigation guidance connects tank selection with solubility and injection duration, and emphasizes accounting for nutrient travel through the system.

5. Choose a refill strategy that operators can sustain

A one-day supply can be a useful starting point, but it is not a universal capacity rule. Map staffing, deliveries and cleaning against the irrigation schedule. The critical period is the longest credible interval when a compatible batch cannot be replenished, including the time needed to mix and verify it.

Prefer identifiable batches where practical. Repeatedly topping up an unknown heel makes stock age and concentration harder to reconstruct. If mixing during dosing is permitted by the design, validate homogeneity through the whole transition; a level rise alone does not prove that the new stock has the intended strength. Define who responds to a late delivery, failed mixer or unavailable operator before relying on the reserve.

6. Size compatible stock groups separately

Do not combine incompatible concentrates to make them fit one tank. Calcium-containing stock commonly needs separation from concentrated phosphate- or sulfate-containing mixtures. Any A/B, micronutrient or acid grouping must follow the actual products, water analysis and agronomist-approved recipe. A large shared vessel does not resolve chemical incompatibility.

Create one demand ledger per stock group, using that channel's own injection rate, event duration, heel and refill interval. If Tank B is used only on selected zones, its consumption need not match Tank A. Keep acid in its specifically designed storage and dosing arrangement where required; fertilizer-tank capacity is not evidence of acid compatibility. Review stock-solution solubility and mixing compatibility before fixing the batch concentration.

7. Check whether the proposed capacity is usable on site

A tank that fits the floor plan may still leave no space to remove a strainer or lift a mixer. Check the delivery route and access to drains before purchase. Commission the level scale and cutoff with a safe, approved procedure. Use clean-water trials where appropriate, then confirm chemical-service performance through the qualified commissioning process.

8. Keep a stock balance that another operator can audit

Reconcile starting stock plus prepared additions minus measured withdrawals against ending stock. Investigate differences larger than the combined measurement uncertainty; do not invent a universal tolerance. Electrical conductivity and pH may flag a mixing problem, but they do not replace an ingredient and volume record. Use EC and pH verification to interpret measurements in their actual sampling context.

9. Diagnose capacity complaints before buying a larger tank

10. Compare one large tank with several smaller tanks

Count redundant capacity only if it can actually serve the same compatible stock without interrupting safe operation. Two tanks cannot simply be added together on paper when each holds a different recipe. Select the arrangement that meets the worst refill window while remaining practical at the smallest expected seasonal batch.

11. Set safe operating boundaries

Follow product labels, safety data sheets and local storage requirements. Have qualified personnel confirm materials, support, containment, backflow protection and control interlocks. Use the specified protective equipment and handling procedure. Isolate energy and pressure before servicing; never enter a stock tank. Do not discharge leftover nutrient stock or cleaning liquid onto soil or into waterways. Provide an approved recovery or disposal route before the first batch is prepared.

The final purchase brief should state stock group, interval demand, justified reserve, measured heel, permitted fill fraction, mixing range and injector duty. It should also identify the refill owner and acceptance checks. That connects tank capacity to a repeatable operation rather than to a catalog number alone.

Image note: the agricultural stock-tank scene was adapted from a real photograph with AI editing. It illustrates storage, suction plumbing and injection equipment; it is not a certified installation drawing or evidence of a field trial.