A/B Fertilizer Stock Tanks: Compatibility, Mixing, and Injection
A clear stock tank at mixing time is not the whole test. The fertilizer must stay dissolved during storage, inject at the intended rate, and deliver the correct nutrient balance after dilution.
For commercial fertigation programs, that starts with separating incompatible ingredients. Calcium belongs apart from concentrated phosphates and sulfates. Potassium silicate needs its own tank and injector. Water quality, stock temperature, and mixing procedure determine whether the rest of the program works consistently.
This guide brings together our fertilizer compatibility reference, solubility table, and AgSil injector guide, with stock-preparation principles from Nutrient Solutions for Greenhouse Crops.
What goes in each stock tank?
A common starting arrangement is:
- Tank A: Calcium nitrate and other ingredients verified as compatible with that stock formulation.
- Tank B: Phosphate and sulfate fertilizers, such as monopotassium phosphate and magnesium sulfate, subject to the complete mixture’s solubility limits.
- Separate silicate tank and injector: Potassium silicate, kept apart from both A and B concentrates.
The tank letters are labels, not a formula. The essential rule is to keep calcium-containing fertilizers out of concentrated phosphate and sulfate stocks. Combining them can form insoluble compounds, remove nutrients from solution, and contribute to clogged filters or emitters.
Some ingredients, including potassium nitrate and magnesium nitrate, can be allocated to either A or B in suitable formulations. That flexibility helps distribute the fertilizer load instead of overcrowding one tank. Confirm compatibility and solubility for the complete recipe before assigning an ingredient.
Place micronutrients by formulation
Sulfate-form micronutrients belong away from the calcium stock. Chelated micronutrients require a different review: the chelating agent, stock pH, and other ingredients affect placement and stability. Do not treat every micronutrient blend as interchangeable.
The greenhouse nutrient manual distinguishes sulfate salts from chelates and warns that overly acidic stocks can compromise chelate stability. Follow the specific product’s technical guidance rather than applying one stock-pH target to every blend.
Potassium silicate requires a separate injection path
Potassium silicate does not belong in either A or B concentrate. Use a dedicated stock tank and dedicated injector, and keep it out of a shared A/B concentrate header.
Our AgSil stock preparation and injector guide describes a silicon-first arrangement: inject silicate into the water stream first, then provide a mixing chamber for dilution before introducing other fertilizer inputs. The point is to prevent concentrated silicate from contacting other concentrates.
Silicate stock preparation also has its own water-quality and pH requirements. Do not apply conventional A/B stock-preparation instructions to the silicate tank. Review the AgSil guide and the product and equipment instructions before setting up the system.
Compatibility and solubility are different checks
Compatibility asks whether ingredients can coexist without an unacceptable reaction. Solubility asks how much will remain dissolved under the actual conditions. Passing one check does not guarantee the other.
Our Fertilizer Solubility Table lists theoretical maximums in grams per 100 mL at the stated temperatures. Use those values as reference points, not recommended stock strengths. They are not guarantees for multi-fertilizer mixtures.
Shared ions can reduce solubility in a blend, so individual fertilizer limits cannot simply be added together. Leave operating margin and validate the complete stock at the lowest temperature it is expected to reach during storage. A batch dissolved in warm water may crystallize after cooling.
Account for the water before calculating fertilizer
Start with a current water analysis. Calcium and magnesium already present in irrigation water contribute to the final nutrient supply and should be included in the recipe calculation. Alkalinity affects pH management; source-water pH alone does not describe that requirement.
For recirculating systems, include nutrients returned in the drain water when calculating fresh fertilizer additions. The target is the composition delivered to the crop, not simply the amount weighed into the tanks.
Keep the preparation-water specification for each stock on the batch sheet. The water used to prepare a silicate concentrate may need to differ from the irrigation source water.
Mix to a final stock volume, not a starting water volume
A recipe for a 100-gallon stock tank means a final solution volume of 100 gallons. It does not mean adding the fertilizers to 100 gallons of water.
For conventional A/B stocks, the greenhouse nutrient manual recommends starting with tanks approximately three-quarters full, adding fertilizers individually with continuous agitation, and allowing time for dissolution before bringing the stock to final volume. Follow the approved recipe’s sequence and the manufacturer’s instructions for the actual products being used.
Use a written batch sheet to record ingredient identity, weight, mixing sequence, final volume, and any required pH checks. Mark off each addition as it is made. This helps prevent both missed ingredients and double additions.
Match concentration to actual injector delivery
Check how the equipment defines its ratio: stock to final solution or stock to water. Also confirm whether the recipe assumes separate delivery rates for A, B, and any auxiliary channel. Do not assume that two injectors labeled 1:100 split one stock dose between them.
For a simple example, if one stock supplies 1% of the final solution volume, delivering 100 mg/L of a nutrient from that stock requires 10,000 mg/L of that nutrient in the stock. This is nutrient concentration, not fertilizer-product weight. The product analysis and all other contributions still have to be included.
Verify actual draw from each stock tank under operating conditions. Check the diluted feed’s pH and EC against the program targets. EC is useful for consistency checks, but it cannot confirm individual nutrient concentrations or prove that A and B are being delivered in the correct proportions.
Validate the program before filling production tanks
- Review the formulation. Confirm nutrient targets, source-water contributions, ingredient placement, and product-specific restrictions.
- Prepare separate small stock batches. Use the intended preparation water, stock strengths, mixing sequences, and final volumes. Never combine the concentrates for a compatibility test.
- Check storage stability. Observe the stocks through the intended holding period and temperature range. Investigate persistent cloudiness, sediment, crystals, or gel formation.
- Check the diluted feed. Use the intended dilution rates and mixing arrangement, including the dedicated silicate path where applicable. Measure pH and EC; use laboratory analysis when verification of individual nutrient concentrations is needed.
- Document the approved setup. Record the recipe, injector delivery, storage conditions, and observations so another operator can repeat the batch.
A clear small batch is a useful screening result, not proof of long-term stability or correct nutrient composition. If solids form, identify the cause before using the stock. Filtering out precipitate does not restore the nutrients that have left solution.
Build a repeatable stock program
Keep calcium separate from concentrated phosphates and sulfates. Give potassium silicate its own tank and injector. Design for the actual water, storage temperature, and injection system, then document the mixing procedure.
For help reviewing ingredient selection, contact Custom Hydro Nutrients with your water analysis, target nutrient profile, proposed stock recipe, tank volumes, and injector specifications. Those details provide a useful starting point for a technical discussion.
Technical references
- Fertilizer Concentrates and Compatibility: separation principles and compatibility charts.
- Fertilizer Solubility Table: temperature-dependent reference values and the common-ion effect.
- AgSil Potassium Silicate and Fertilizer Injector Concentrates: silicate stock preparation and dedicated injection arrangement.
- Nutrient Solutions for Greenhouse Crops, edited by Geerten van der Lugt (2016), especially Chapter 1, “Water Quality and Analyses,” Chapter 8, “Fertiliser Recipe Calculation,” and Chapter 9, “Preparing Highly Concentrated Stock Solutions.” The manual is linked from our compatibility reference page.
