- Explain why alkalinity, not pH, sets how much acid or base a solution needs.
- Match each common acid and base to the nutrient it adds and to G6984’s handling notes.
- Run a small-batch titration to find your own dose and scale it to the reservoir.
01 / Alkalinity sets the dose, pH sets the direction
Penn State’s irrigation-water guidance defines alkalinity as the dissolved carbonates, bicarbonates and hydroxides, reported as mg/L of calcium carbonate, that buffer or neutralize acids. University of Missouri G6984 adds that alkalinity does not correlate with pH and cannot be measured with a pH meter: “The higher the alkalinity, the more acid you will need to make a change in pH.”
Penn State calls roughly 30–100 mg/L ideal, up to 150 mg/L suitable for many plants, and below 30 mg/L no buffering at all. G6984’s source-water table lists 40–160 ppm CaCO₃ equivalent; above 160 ppm more acid is needed, below 40 ppm pH changes constantly.
Reading: Interpreting Irrigation Water Tests ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗
02 / Acids and bases: what each one leaves behind
G6984 lists the acids for lowering pH as citric or acetic, phosphoric, nitric and sulfuric, from most to least expensive. UMass’s pH and alkalinity fact sheet says phosphoric and nitric acid add P and nitrate, so the fertilizer program may need modifying. Sulfuric acid contributes sulfate; Penn State names it for treating scale-forming hardness. Citric and acetic are organic acids, and OSU includes vinegar; because microbes can metabolize organic acids, this guide’s own expectation, not a finding in the cited documents, is that their effect fades sooner than a mineral acid’s. On handling, G6984 rates citric/acetic safer than phosphoric and sulfuric, and nitric the most dangerous.
For raising pH, G6984 names potassium bicarbonate or potassium hydroxide, which provide potassium, and warns off sodium bases (sodium can be toxic to plants) and lime (low solubility, injector wear); OSU adds potassium carbonate. Penn State’s alkalinity definition covers bicarbonate, carbonate and hydroxide, so each of these bases raises the buffer as well as the pH; so does the next top-up of high-alkalinity water. OSU lists bottled pH down and pH up products without saying what is in them; read the label, because its active ingredient and concentration decide which nutrient you add and how far to dilute.
| Adjuster | Direction | Adds to the solution |
|---|---|---|
| Phosphoric acid | Lowers pH | Phosphorus (P) |
| Nitric acid | Lowers pH | Nitrate nitrogen |
| Sulfuric acid | Lowers pH | Sulfate (S) |
| Citric or acetic acid | Lowers pH | Organic acid (no mineral nutrient) |
| Potassium hydroxide | Raises pH | Potassium (K) and alkalinity |
| Potassium bicarbonate or carbonate | Raises pH | Potassium (K) and alkalinity |
| Sodium bases; calcium carbonate | Avoid | Sodium; low-solubility lime |
Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Water Quality: pH and Alkalinity (Greenhouse and Floriculture fact sheet; no publication number shown on the page) ↗ · Interpreting Irrigation Water Tests ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗
03 / Find your own dose with a small-batch titration
No table can print millilitres per litre: the answer depends on your water’s alkalinity, the acid and its concentration, and the fertilizer already dissolved. G6984 points growers to a calculator fed by a water test; UMass says the amount is determined by laboratory titration with the appropriate acid, then fine-tuned once dosing starts. The bench procedure below is original practice, not a published protocol.
Add acid to water, never water to acid, and wear eye protection; that is general handling practice, not from the cited documents. Do not pour concentrate into a tank with plants in it; that is this guide’s rule. OSU’s instruction is to add slowly, wait several minutes before adding more, and repeat until the pH reaches the range.
- Make a working dilution, say one part concentrate in nine parts water; label it with ratio and date.
- Draw exactly 1 L of mixed nutrient solution, not plain source water, since dissolved fertilizer changes the starting pH and buffering; record temperature and pH with a calibrated meter.
- Add the working dilution by graduated syringe in small increments, stir, wait for a stable reading, and log millilitres against pH.
- Stop at your target pH and note the total millilitres per litre.
- Multiply by the reservoir’s actual volume, not its label capacity, dose the circulating tank and remeasure after full mixing.
- Repeat the titration whenever source water, recipe or concentrate changes, and check pH again the next day.
Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Water Quality: pH and Alkalinity (Greenhouse and Floriculture fact sheet; no publication number shown on the page) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗
04 / Drift: after mixing and in operation
G6984 states that solution pH and EC change as fertilizers are added and as plants and microbes use water and nutrients over time. Right after mixing, OSU’s procedure has you stir and allow the reading to stabilize, which may take a couple of minutes, before correcting. In operation, OSU says water above 75 ppm alkalinity causes pH to increase and needs more frequent checks; below the section 01 floors, Penn State and G6984 report no buffering and constantly changing pH.
Rising pH in a topped-up reservoir points at alkalinity; falling pH can follow a recipe heavier in ammonium, the reverse of the high-nitrate fertilizers G6984 lists among pH raisers. OSU’s order: set EC first, then pH, and check both daily at the same time.
Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗ · Interpreting Irrigation Water Tests ↗
05 / Worked example: two waters, one pH
Two growers read pH 7.6 on their mixed solution; one has source-water alkalinity of 30 mg/L as CaCO₃, the other 120 mg/L. Penn State’s table puts the first at the edge of no buffering and the second above the 100 mg/L level of concern, where acid injection is the listed treatment.
FIELD QUESTIONYour 1 L titration of mixed solution took 0.8 mL of a 1-in-10 working dilution of phosphoric acid to reach pH 6.0. The reservoir holds 60 L. How much do you dose, and what else has changed?
Dose scales with volume, 0.8 mL/L × 60 L = 48 mL of the working dilution, or 4.8 mL of concentrate. Phosphoric acid supplies phosphorus (UMass), so the solution now carries more P than the recipe gave it; log the dose. The 0.8 mL is a measurement for this water and recipe, not a figure to reuse.
Reading: Interpreting Irrigation Water Tests ↗ · Hydroponic Nutrient Solutions (G6984) ↗
Parts & buying criteria
Build your own parts & cost worksheet →
Showing ebb and flow. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the pump intake or lower roots; freeboard is deliberate.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Pump & supply1 · illustrated quantity+
Moves nutrient solution from the reservoir to the growing area.
Inspect: Trace installed lift and branching; the pump label does not prove delivered flow.
Maintain: Measure output after service and inspect intake, filter and minimum water level.
Pump curve matched to installed head, required pressure and service needs.
Check size, materials and operating conditions with your chosen supplier before ordering.03Growing tray1 · illustrated quantity+
Contains supported media pots above the lower reservoir.
Inspect: Inspect the flood level, pot bases and accessible drain position.
Maintain: Verify full drainage and inspect moisture in representative pots.
Rigid cleanable tray with independent overflow and adequate support.
Check size, materials and operating conditions with your chosen supplier before ordering.04Root-zone mediumFor illustrated containers · illustrated quantity+
Stores moisture and air around the roots in the illustrated containers.
Inspect: Compare the medium surface with irrigation and drainage.
Maintain: Inspect wetting at several depths; prevent fines from reaching drains and emitters.
Prepared, stable grade matched to the irrigation method and crop.
Check size, materials and operating conditions with your chosen supplier before ordering.05Drain & overflow1 · illustrated quantity+
Returns drainage to the reservoir and provides a controlled high-water route.
Inspect: Follow the full slope and look for the separate high-level opening.
Maintain: Test pump-off drain-back and keep roots and medium out of the outlet.
Accessible fittings and verified capacity under normal and fault conditions.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- Penn State ExtensionInterpreting Irrigation Water Tests ↗
Distinguishing alkalinity, pH, salts and source-water problems.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- Oklahoma State University ExtensionElectrical Conductivity and pH Guide for Hydroponics ↗
Water analysis, salinity, pH and monitoring.
- UMass Extension · Center for Agriculture, Food, and the Environment, University of Massachusetts AmherstWater Quality: pH and Alkalinity (Greenhouse and Floriculture fact sheet; no publication number shown on the page) ↗
Prepared by Douglas Cox, August 1995; page last updated 16 June 2026. Supports: alkalinity as the water’s ability to neutralize acidity, reported as ppm CaCO₃; a pH test by itself not indicating alkalinity; growers injecting phosphoric, nitric or sulfuric acid into high-alkalinity water; acids being dangerous to handle and possibly damaging injectors and piping; phosphoric and nitric acid as sources of P and NO₃ that may require the fertilizer program to be modified; and the acid requirement being determined by laboratory titration of a water sample with the appropriate acid or by calculation, with fine-tuning once injection starts.
The next useful connections.
Nutrient mixing: from grams to elemental ppm
Scale two documented fertilizer examples, read the bag analysis and calculate what actually enters the final solution.
Interactive tool · 6 min + guided practice →Nutrients & mixingEC, pH and the meter “ppm” scales
Calibrate your instruments, record unambiguous units and interpret concentration without pretending EC is a nutrient analysis.
Interactive tool · 6 min + guided practice →Nutrients & mixingReservoir management and solution replacement
Use a water-and-nutrient ledger to distinguish top-ups, rebalancing and a complete batch change.
Interactive tool · 6 min + guided practice →Cycling & water balanceWater quality: pH, ammonia and alkalinity
Track the interacting measurements that govern fish, roots and nitrification.
Interactive tool · 6 min + guided practice →Source waterSource water and alkalinity
Read a laboratory report, distinguish equivalent units from elemental nutrients and prepare water deliberately.
6 min + guided practice →