FoxFarm Grow Big Review: pH & EC Test Results

Last updated: August 2026

Quick answer: HWGrow’s controlled screen added 1 mL of FoxFarm Grow Big 6-4-4 to 500 mL of reverse-osmosis water. That equals 2 mL/L, or about 7.6 mL per U.S. gallon. The sample’s pH fell by about 0.75 unit on both pH meters, and the video reported roughly 620–630 PPM after mixing. Those are useful observations for that water, bottle and test—not a cannabis feed schedule or proof that FoxFarm’s current labeled dose is universally too strong.

Video chapters: Grow Big pH and PPM test

1. 0:00 — Grow Big 6-4-4 label overview
2. 0:27 — Labeled dose discussion
3. 0:35 — Initial pH readings
4. 0:51 — 500 mL water plus 1 mL Grow Big
5. 1:11 — Final pH readings
6. 1:31 — Conductivity-based PPM comparison
7. 1:59 — Dose extrapolation; limits explained below
8. 2:24 — Original feeding conclusion; corrected below

Selecting a timestamp opens the video at that moment without moving you away from this section.

First, confirm which Grow Big bottle you have

This test used FoxFarm Grow Big Liquid Plant Food 6-4-4, the soil-formula concentrate. FoxFarm also sells Grow Big Hydroponic 3-2-6. The names look similar, but the formulas and instructions are not interchangeable.

FoxFarm’s current product page still lists the soil formula as 6-4-4 and describes it as a vegetative-growth concentrate with micronutrients. The current general direction is 10–15 mL per gallon every other watering, with separate instructions for heavy and foliar feeding. Always confirm the label on your bottle because formulas and schedules can change. Read the current FoxFarm Grow Big label guidance.

HWGrow Grow Big test results

The ratio matters. The video did not test 1 mL per gallon; it tested 1 mL in half a liter. The separate HWGrow nutrient-test protocol documents this standardized 500 mL comparison method and its limits.

What the result really tells you

Grow Big lowered pH in low-buffer RO water

Both pH meters moved by almost the same amount, which is a useful consistency check. It does not mean every tap-water or full-feed mixture will drop by 0.75. Reverse-osmosis water has very little alkalinity, so its pH can move sharply when a concentrated fertilizer is added. Your water source and the rest of the recipe change the final result.

Grow Big added substantial conductivity

The PPM display rose because dissolved fertilizer ions increased electrical conductivity. That shows the concentrate contributes a meaningful portion of the solution’s total ionic strength. It does not identify the concentration of nitrogen, phosphorus, potassium or each micronutrient, and it does not prove that one dose is safe or excessive for every plant.

The two instruments were not two independent samples

Both conductivity meters read the same mixed sample. That compares instruments; it does not replicate the preparation. The video manually added 50 PPM to one meter because it was “always off.” A modern repeat should calibrate or verify both meters against a conductivity standard, report the conversion scale and repeat independently prepared samples. A fixed correction should never be guessed from habit.

Why the old “never more than 10 mL” conclusion was too strong

The source video extrapolated its 7.6 mL-per-gallon equivalent result to higher doses and concluded that more than 10 mL during vegetative growth would never be needed. The same video called FoxFarm’s 10–15 mL label direction “insane.” The experiment cannot establish either claim.

• FoxFarm’s current label still lists 10–15 mL per gallon for general feeding.
• The video measured a single bottle in low-EC RO water, not a complete FoxFarm recipe in a planted container.
• Conductivity can be roughly proportional across a limited range, but a simple extrapolation is not a plant-response trial.
• The original PPM scale was not documented in the video, and one reading included a manual correction.
• Medium charge, water alkalinity, plant size, stage, irrigation frequency and the other products in the program all change the decision.

The fair conclusion is simpler: measure the complete mix you actually plan to use. Compare it with the current label, your medium, your input/runoff trend and the plant’s response. Do not copy HWGrow’s historical personal dose or the bottle maximum as a universal cannabis recipe.

How to use Grow Big without guessing

1. Identify the formula. Confirm soil 6-4-4 versus hydroponic 3-2-6 before measuring anything.
2. Read the current label and schedule. Use the instructions for your exact product and growing program; do not combine unrelated schedules.
3. Measure the source water. Record temperature, pH, EC and alkalinity information when available. Search your ZIP code plus “water report” for the local utility report.
4. Mix accurately. Use a syringe or graduated tool, add each concentrate to water separately and stir before the next product. Never mix concentrates together.
5. Measure the complete solution. Record final pH and EC; label the PPM scale if you use that display.
6. Adjust pH last. Let the mixed feed stabilize before deciding whether adjustment is necessary.
7. Feed the plant and medium, not a calendar. Use pot moisture, plant stage, label guidance, visible response and repeatable runoff trends.

The nutrient mixing-order guide explains why concentrates must be diluted one at a time. For container irrigation and runoff, use the fabric-pot feeding guide.

Grow Big test limitations

Recommended FoxFarm and testing supplies

Related HWGrow comparison: For the larger three-part lineup and workflow differences, see General Hydroponics versus the FoxFarm Trio.

FoxFarm Grow Big 6-4-4The soil-formula concentrate tested here. Confirm 6-4-4 on the current bottle and follow its current label rather than a copied schedule.
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FoxFarm Soil Liquid TrioGrow Big is normally used as one part of a broader program. Mix each product into water separately and measure the complete solution.
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pH and EC meter kitMeasure the water baseline, complete recipe and runoff. Calibrate the probes and record the EC or named PPM scale.
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Affiliate disclosure: HWGrow may earn a commission from qualifying purchases at no additional cost to you. Product destinations may be replaced with equivalent items; always verify the exact formula and label before use.

Want the complete feeding workflow?

This review shows what one bottle did in one controlled sample. The Grow With Me Masterclass puts mixing, watering, runoff and plant response into a complete workflow without reproducing protected lesson content here. If your result differs, ask HWGrow a question and include the exact bottle, lot, dose, water source, complete recipe, temperature, pH, EC/PPM scale, medium and plant stage.

FoxFarm Grow Big FAQ

Did HWGrow test 1 mL of Grow Big per gallon?

No. The test used 1 mL in 500 mL of water, which equals 2 mL/L or about 7.6 mL per U.S. gallon.

Does Grow Big always lower pH by 0.75?

No. That was the change in the tested low-buffer reverse-osmosis water. Tap-water alkalinity and the rest of the nutrient recipe can change the result.

Does 620 PPM mean the dose is too strong?

Not by itself. The PPM scale, source-water baseline, medium, plant stage and complete recipe must be known. The historical test also applied a manual offset to one meter.

Is Grow Big 6-4-4 the same as Grow Big Hydro 3-2-6?

No. They are different formulations with different labels. This page covers the Grow Big Liquid Plant Food soil formula, 6-4-4.