Fertigation Stock Solutions: Check Solubility and Mixing Compatibility
Fertigation stock solutions are concentrated fertilizer mixtures that an injector dilutes into irrigation water. Before making a batch, check the exact product specifications, the water analysis, the lowest expected storage temperature, and compatibility at both stock and irrigation concentrations. A fertilizer that dissolves by itself may still react with another fertilizer or with minerals already in the water.
Solution tanks used with fertigation equipment. Photo: IrriNex.
The useful output is a batch record that says what was evaluated and under which conditions. This guide provides that record, a concentration example, and a two-stage compatibility check. For the wider equipment and timing context, see the IrriNex guide to fertigation in drip irrigation. The procedure below starts with a nutrient programme already approved for the crop; it does not prescribe fertilizer doses or chemical treatment.
Start with a stock-solution specification sheet
Record each product separately before deciding whether products can share a concentrate. A nutrient analysis on the bag identifies nutrient content, but it does not by itself establish solubility, permitted mixing partners, or storage stability. Obtain the technical sheet and safety data sheet for the actual formulation and grade. A familiar trade name or matching nutrient analysis is insufficient evidence when the formulation has changed.
Use the following worksheet for a proposed batch. Keep the supporting document beside the recorded value, so another operator can distinguish a measured condition from an assumption. Leave an unknown entry marked “pending”; an empty cell should never become an implied approval.
| Record | What to enter | Decision it supports |
|---|---|---|
| Product identity | Exact formulation, grade, lot, label and technical-sheet revision | Whether the evidence applies to this material |
| Concentration basis | Product mass, final solution volume, and units used by the specification | Whether the proposed concentration and published limit are comparable |
| Temperature and holding period | Mixing temperature, lowest expected stock temperature, intended storage duration | Whether cold storage and delayed use have been considered |
| Water identification | Source, sample date, treatment state, laboratory report and temperature | Whether the test water represents the batch and irrigation supply |
| Mixing restrictions | Permitted partners, exclusions, concentration limits and documented preparation method | Whether a small mixture test is appropriate at all |
| Stock observation | Test identity, actual quantities, temperature, elapsed time, clarity and solids | Whether the candidate remains physically stable under the tested storage conditions |
| Dilution observation | Irrigation-water sample, intended dilution, observation method and result | Whether a separate compatibility problem appears after injection |
| Release decision | Approved, hold or rejected; responsible person and unresolved questions | Whether the batch can enter the operating procedure |
Also distinguish a prepared stock reservoir from a pressure bypass fertilizer tank whose contents change as irrigation water passes through it. Do not assume either arrangement delivers a constant concentration merely because the tank has a volume marking. Identify the actual preparation and withdrawal method on the worksheet; the chemistry assessment must describe the liquid that the equipment will handle.
Read the water analysis before selecting the concentration
Water contributes dissolved substances before any fertilizer enters the tank. The UMass Extension water-analysis guidance includes pH, electrical conductivity, alkalinity, calcium and magnesium among the useful laboratory measurements. Ask the laboratory which additional measurements the proposed fertilizer combination requires, including sulfate or phosphorus where relevant.
Attach the report for the actual water used to prepare stock. If the concentrate is made with treated water but injected into a different supply, record both waters. Write alkalinity and hardness with their reported units and analytical basis; neither is interchangeable with a pH reading. A single pH value does not describe all the dissolved ingredients that can react in a mixture.
Record source changes, blending and treatment that occurred after sampling. A report from one well does not establish the chemistry of a later blend. Reconcile uncertain or unexpected results with the laboratory or fertilizer specialist before making the batch. This worksheet deliberately leaves the acceptance limits to the product and site assessment; it does not convert greenhouse water-quality ranges into universal stock-solution limits.
Check solubility at the relevant storage temperature
Solubility data are meaningful only when the product, temperature and concentration basis match the proposed use. The University of Georgia discussion of fertilizer solubility warns that cooling can bring dissolved fertilizer out of solution and that pure-water solubility values do not transfer directly to mixtures containing other fertilizers.
Compare the proposed stock with data covering its lowest expected temperature during storage and withdrawal. Do not size a batch around how much material can be dissolved in heated water. If the relevant temperature is outside the available specification, obtain more information or a revised, supported formulation. An operator should not invent a percentage reduction and treat it as a verified safety margin.
Keep product mass and final solution volume explicit
The following invented example checks units only. It does not identify a fertilizer, establish its solubility, or approve a mixture. Suppose a proposed record specifies 12 kg of one dry product in a final stock volume of 150 L. The arithmetic is:
| Quantity | Calculation | Result |
|---|---|---|
| Product mass | 12 kg × 1000 g/kg | 12000 g |
| Stock concentration | 12000 g ÷ 150 L final solution | 80 g/L of product |
| Scale factor for a 0.5 L final sample | 0.5 L ÷ 150 L | 1/300 of the batch |
| Equivalent sample product mass | 12000 g ÷ 300 | 40 g in 0.5 L final solution |
The denominator is final solution volume. Adding 12 kg to 150 L of water does not necessarily produce 150 L of solution. Likewise, a specification in grams per litre of water cannot be compared directly with this example's grams per litre of final solution. Resolve that basis with the supplier rather than silently substituting one denominator for the other.
The value 80 g/L describes the fertilizer product, not the concentration of nitrogen or another nutrient. A liquid product specified by mass also needs an appropriate density before its mass can be inferred from volume. For the separate task of translating a crop nutrient target into delivery settings, use the fertigation injection-rate calculation worksheet.
If a permitted candidate contains several products, scale each product independently and preserve the specified final volume and preparation method. Adding the individual concentrations does not establish mixture solubility. The small-sample quantity above is an arithmetic illustration; the responsible specialist should choose the actual test size, equipment and method.
Check mixture compatibility separately from solubility
Two soluble fertilizers do not automatically make a stable concentrate together. UMass Extension's nutrient-management guidance identifies calcium with sulfate or phosphate, and some iron compounds with phosphate, as common incompatibility concerns in highly concentrated stock solutions.
This is a reason to check the actual formulations and keep incompatible concentrates separate. It is not a universal tank recipe based on nutrient names. “A” and “B” are tank identifiers, not chemical guarantees: the assignment of any product depends on its specification and its partners. Do not infer that every calcium product behaves identically, or that everything without calcium belongs in one compatible mixture.
Even when separate stock solutions are appropriate, their intended diluted combination still needs evaluation with the irrigation water. Ask where the concentrates first meet and whether the arrangement provides the dilution assumed in that evaluation. Document unresolved equipment questions for the system designer. This compatibility record does not replace an engineered multiple-tank layout or authorize mixing concentrates together at a common suction connection.
Run separate stock and irrigation-water checks
A small test screens a candidate that has already passed the document review. It is not a way to discover whether an unknown chemical combination is safe to handle. Follow the labels, safety data sheets and a suitable specialist-approved method. Exclude prohibited mixtures and any uncertain acid, disinfectant or pesticide combination from this fertilizer worksheet.
Check the concentrate under its intended holding conditions
- Identify the permitted candidate, source water, target concentrations, temperature range and intended holding period. Label the test container and record the actual quantities and preparation sequence required by the product instructions.
- Observe whether the ingredients dissolve as expected using the approved method. Record undissolved material, new cloudiness, crystals or sediment. Do not add extra chemicals or change the concentration simply to make the sample look clear.
- Keep and observe the sample under the agreed storage conditions for the planned holding assessment. Record temperature and elapsed time at each observation, including the colder conditions that the full batch may encounter.
Distinguish “clear immediately after mixing” from “remained clear over the evaluated holding period.” If only the first observation exists, the record does not establish the second. Preserve any failed result and the original quantities, rather than overwriting them when a revised candidate is tested.
Check the intended dilution with irrigation water
The UF/IFAS guidance on fertigation clogging describes checking for cloudiness at the approximate application dilution for about 2 hours. That diluted-water screen is different from testing a concentrate for storage. Use the intended operating water and a documented dilution; a clear sample made with unrelated clean water answers a different question.
Where multiple stocks will be delivered, the method must represent their intended diluted combination without directly combining incompatible concentrates. Have the fertilizer specialist specify the preparation method. Record what was tested and avoid extending an observation beyond its conditions: a short clear test cannot certify an entire season, every water source, or the absence of all microscopic deposits.
Interpret a failed observation without guessing the cause
Appearance helps decide whether to hold a candidate; it usually does not identify the solid. Use the observation and the batch history to frame the next question, rather than choosing a corrective chemical from colour alone.
| Observation | Question to investigate | Batch decision |
|---|---|---|
| Material remains after preparation | Wrong product, incomplete dissolution, excessive concentration or unsuitable preparation conditions? | Hold and review the product and method |
| Crystals appear after cooling | Does the supported concentration cover the actual minimum temperature? | Hold and obtain a supported revision |
| Separate samples are clear; the permitted combination turns cloudy | Is there an interaction between the products or with the water? | Reject that combination pending specialist review |
| Stock is clear; irrigation dilution develops solids | Does irrigation-water chemistry differ from the stock-preparation water? | Hold injection and review the diluted combination |
Filtering out a precipitate does not restore the original dissolved nutrient composition. Do not assume the remaining liquid still matches the intended feed. Keep the affected batch out of use until its disposition is resolved. Inspect the irrigation system separately if suspect material has already been injected; the drip maintenance and monitoring guide provides the wider maintenance context.
Release a defined batch and keep its evidence
A release record should identify the exact products, concentrations, waters, temperature range, holding period and preparation method reviewed. Attach the observations and the responsible person's decision. If a limit or compatibility question remains unresolved, the correct outcome is a hold, even when the arithmetic is correct.
Revisit the record when the product, source water, concentration, storage period or relevant temperature changes. Retain previous versions so operators can see what changed. Electrical conductivity and pH can support operating checks where the product procedure calls for them, but neither measurement alone establishes the identity or compatibility of every ingredient.
For a system discussion, bring this completed worksheet to the IrriNex fertigation solution review, together with the proposed tank and injector arrangement. The practical deliverable is a traceable stock specification with explicit conditions, allowing chemistry questions and equipment questions to be resolved before the full batch is prepared.
