Plant Nutrition & Mixing Nutrient Solutions
Plant Nutrition & Mixing Nutrient Solutions
Plants need the right nutrients, in the right ratio, at the right time. This guide covers both halves of that: what plants actually need and how they take it up, then how to mix a tank accurately so those minerals stay available.
What Plants Actually Need
Primary macronutrients (NPK)
The big three, and the ones printed as a ratio on every bottle:
- Nitrogen (N) — leaf and stem growth, proteins, chlorophyll, enzymes. Most important in veg.
- Phosphorus (P) — photosynthesis, energy transfer, root growth, fruit and flower formation, seed quality.
- Potassium (K) — enzyme activation, sugars, stem strength, water regulation, fruit flavour and colour.
As a rule, grow feeds are higher in N; bloom feeds are higher in P and K. The exact ratio varies by brand.
Plants need three more macros — oxygen, carbon and hydrogen — but they take those from air and water, so they’re not something you dose.
Secondary macronutrients
Needed in slightly smaller amounts: magnesium (Mg), sulphur (S), calcium (Ca) and silicon (Si).
Micronutrients (trace elements)
Needed, but only in tiny amounts: sodium (Na), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), chlorine (Cl), nickel (Ni) and cobalt (Co). A complete feed will cover these.
How Plants Absorb Nutrients
Through the roots
Roots take up whatever is dissolved in your media or solution. Two things govern how well that works:
- pH. At the wrong pH some nutrients simply can’t be absorbed, however much you add. Stay between 5.5 and 6.5.
- Strength. Too weak and you underfeed; too strong and you overfeed and risk burn.
Through the leaves
Leaves mostly take oxygen, carbon and hydrogen from the air, but they absorb sprayed nutrients almost instantly. That makes foliar feeding useful for:
- Correcting a deficiency quickly
- Feeding when roots are damaged
- Feeding cuttings and seedlings
Where a deficiency shows tells you what kind it is
Some nutrients move around the plant more easily than others, and that gives you a fast diagnostic.
- N, P and K are mobile. When the plant runs short it moves them from older leaves up to new growth. So damage on older leaves points to a macronutrient problem.
- Micronutrients are far less mobile. They stay put in older leaves. So damage on newer leaves points to a micronutrient problem.
For symptom-by-symptom diagnosis and fixes, see the troubleshooting guide.
Reading an NPK Ratio
Plants need all three, but the proportions change with growth stage. That’s why you get separate formulas:
- Grow nutrients: high in N and P
- Bloom nutrients: high in P and K
- ‘Baby’ / start nutrients: very high in N, and a little higher in K than a grow feed
Coco feeds don’t split into grow and bloom. Coco holds onto nitrogen and releases potassium, so coco‑specific feeds are formulated differently to account for it. In coco you shift the balance by adjusting dosage and adding boosters, not by swapping bottles.
Boosters change your ratio, not just your strength
When you add a booster you aren’t only adding more nutrient — you’re changing the proportions. A PK booster is high in phosphorus and potassium, so it widens the gap between N and PK.
That’s why a PK booster is usually introduced from week 4 of flowering: vertical growth has stopped so nitrogen demand falls, and the plant is setting its first flowers, which extra P and K drives hard.
Working out your actual NPK in solution
If you want to know what your tank really contains, do this for every bottle you add:
- Find the NPK ratio on the label.
- Work out the dilution factor: 1 L ÷ dose in ml, converted to ml. At 4 ml/L that’s 1000 ÷ 4 = 250.
- Divide each figure by the dilution factor.
| Worked example — a bloom feed at 4 ml/L, NPK 4.13 / 3.46 / 6 |
|---|
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Repeat for each additive and add them together. Adding a PK booster contributing 0 / 0.00450 / 0.00900 gives a combined 0.01652 / 0.01834 / 0.03300 — so that single booster leaves you with roughly twice as much potassium as nitrogen. Worth knowing before you add a second one.
pH Management
pH measures how acidic or alkaline your solution is. Plants absorb minerals best in a slightly acidic range. Keep this front of mind every time you mix a feed — it’s your reference point for nutrient availability.
Target Range
- Good pH: 5.6 – 6.8
- Optimum pH: ~6.2
Use a calibrated digital pH meter (e.g. Aqua Master pH Pen). Check after mixing nutrients and adjust with pH Down/Up in small amounts.
Element Availability Across pH
At ~pH 6–6.5, the “big three” (N, P, K) and key secondaries (Ca, Mg, S) are widely available, while most traces (Fe, Mn, B, Cu, Zn, Mo) also sit in their usable bands. Drift too alkaline and iron/manganese availability falls; too acidic and calcium/magnesium uptake suffers. Phosphorus deficiency in particular is most common above pH 7 or below 5.5.
CF / EC Control (Nutrient Strength)
CF (Conductivity Factor) or EC shows how much dissolved mineral is in your tank — i.e., the strength of your feed. It should be your second key reference after pH to avoid under/overfeeding.
Typical CF Targets
- Cuttings & seedlings: CF 6–8
- Vegetative growth: CF 8–12
- Flowering/fruiting: CF 12–16
Measure with a calibrated CF/EC meter (e.g. Bluelab Truncheon). Record readings so you can repeat success from batch to batch.
Hard or Soft Water?
Hard water has more dissolved calcium/magnesium (often visible as limescale); soft water has less. Choose a nutrient line formulated for your water type, or use an RO filter in very hard areas.
How to Check
- Fill a bucket with tap water and leave it to stand for 24 hours (room temperature, chlorine dissipated) or use Ecothrive Neutralise to instantly dechlorinate water.
- Take baseline readings with calibrated meters:
| pH Reading | CF Reading | Recommended Feed Type |
|---|---|---|
| 7.8 or above | 8 or above | Hard Water Feed |
| 7.7 or below | 7 or below | Soft Water Feed |
Quick Guide to Meter Readings
Baseline CF varies with water hardness. Use this as a simple reference when aiming for quarter/half/three‑quarter/full strength feeds.
| Tap Water | ¼ Strength | ½ Strength | ¾ Strength | Full Strength | |
|---|---|---|---|---|---|
| Typical CF in hard water | 8 | 12 | 16 | 20 | 24 |
| Typical CF in soft water | 2 | 6 | 10 | 14 | 18 |
Feeding Schedule by System (Guide)
Use the strengths below as a week‑by‑week starting point. Always watch plant response and adjust in small steps.
| Stage → | Rooting Cuttings & Starting Seeds | Vegetative Growth | Flowering/Fruiting Growth | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Week → | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| NFT | ¼ | ½ | ¾ | Full | ¾ | ½ | |||||
| Flood & Drain | ¼ | ½ | ¾ | Full | ¾ | ½ | |||||
| Hydro Pots | ¼ | ½ | ½–¾ | ¾ | ¾ | ½ | |||||
| Coco | ¼ | ½ | ½–¾ | ¾ | ¾ | ½ | |||||
| All‑Mix / well‑fertilised compost | Water | Water (feed once if needed with ½‑strength grow) | ½–¾ every other watering | ||||||||
Switch from grow to bloom nutrients when you change from 18/6 to 12/12 (or within a week). Full‑strength feeds are for big, heavy‑feeding plants in active recirculating systems (e.g., NFT). In pots, it’s rarely necessary to exceed three‑quarters strength.
Most bottles list a “normal/half/full strength” dilution rate; some include a specific “seedling strength” too—use those as your product‑specific reference alongside the table above.
How to Create a Nutrient Solution
1) Calibrate and Prepare
Calibrate pH and CF/EC meters following manufacturer instructions. Accuracy here saves headaches later.
2) Take a Background Reading
Fill your tank with tap water that’s stood for 24 hours. Measure and note the background CF/EC and pH before you add anything.
3) Add Nutrients and Set Strength
Dose nutrients to the appropriate strength for your stage/system, mixing thoroughly between parts. Account for the background CF — additives can nudge readings up. If the solution is too strong, add more water.
4) Set pH
With the solution circulating in the system, check pH for a “real” reading. As a guide, aim around pH 6.3–6.6 for NFT, Flood & Drain, and DWC. If pH is high, add small drops of pH Down to correct.
Best Practice & Tips
- Replace the whole tank regularly — weekly works well for most growers — rather than endlessly topping up.
- Keep nutrient temperature stable (roughly 18–21 °C) for root health and oxygen levels.
- Oxygenate tanks with air stones or circulation to keep solutions fresh.
- Record pH/CF and volumes for repeatable results.
- Store nutrients somewhere frost‑free so they don’t precipitate out.
This blog is part of our How to Grow Indoor Plants – Your Complete Guide series, designed to tell you everything you need to successfully grow indoors.
