Short answer. "Soil" indoors means a bagged potting mix (peat or coco plus compost, perlite and a starter charge of fertiliser), not garden dirt. It is the most forgiving system: the mix buffers pH and holds nutrients and water for days. Water at pH 6.2–6.8, feed lightly (EC 0.8–1.6 mS/cm, i.e. 400–800 ppm500 / 560–1,120 ppm700) and only once the mix's own charge runs out, and water when the pot is light, to 10–20 % runoff.
How it works
The mix has three jobs: hold water, hold air, and hold nutrients on its exchange sites (cation exchange capacity). Compost and peat buffer pH, so a slightly off irrigation pH is absorbed for weeks. Roots need air as much as water: a saturated pot has little air-filled porosity and roots suffocate. That is why over-watering looks like wilting [1].
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Irrigation pH | 6.2–6.8 | Jardin "Soil (bottled feed)" band 6.3–6.8 [4] |
| Feed EC (when feeding) | 0.8–1.6 mS/cm | Jardin caps soil at ≤ 1.2 as default [4]; never hydro-strength |
| Runoff EC | Within ~0.5–1.0 of input | Rising runoff EC = salt build-up |
| Pot size, photoperiod | 11–25 L (3–7 US gal) final | Bigger pots dry slower and yield more within reason; smaller pots work if you water and feed more often (see note) |
| Pot size, autoflower | 11–15 L (3–4 US gal), plant directly | Avoid transplant stalls |
| Watering trigger | Pot ~40–50 % of its just-watered weight above dry weight | Learn by lifting |
| Root-zone temperature | 18–24 °C (64–75 °F) |
House position: pot size is a trade-off. Bigger pots give bigger yields, within reason; across many crops, bigger rooting volume generally means bigger plants (Poorter et al. 2012 meta-analysis). But root volume isn't the whole story: the founder has grown a QP+ (4 oz+, ~115 g+) from a 16 oz (~0.5 L) cup. If you push water and nutrients hard enough (high-frequency fertigation), big things come in small packages. Small pots need more frequent feeding and watering and leave less margin for error; big pots are more forgiving but dry slower. Match the pot to how often you can irrigate.
How to run it
- Mix: a quality potting mix with 20–35 % perlite or pumice. Garden soil compacts and carries pathogens [3].
- Start: the mix's starter charge usually feeds seedlings for 2–4 weeks. Water only, pH-adjusted.
- Water: when the pot is light and the top 2–3 cm (1 in) is dry. Slowly, until 10–20 % runs out. Discard runoff; don't let pots sit in it.
- Feed: begin at half the label rate when lower leaves start to pale; feed every second or third watering ("feed, water, feed" is common).
- Check monthly: pour-through or runoff EC/pH. If runoff EC climbs well above input, do a root-zone flush: 2–3× the pot volume of pH-adjusted water (pH ~6.3–6.8), slowly, then check runoff EC and go back to normal feeding.
Failure modes
| Problem | Sign | Fix |
|---|---|---|
| Over-watering | Droopy, firm-looking leaves, wet heavy pot, slow growth, fungus gnats | Water less often, more air in mix, fabric pots |
| Under-watering | Limp, papery droop, very light pot | Water slowly in stages so the mix re-wets |
| Salt build-up | Burnt tips, runoff EC high | Root-zone flush (2–3× pot volume), then normal feed |
| pH drift | Mixed deficiencies despite feeding | Check runoff pH; adjust water pH, not with harsh swings |
Jardin notes
Jardin medium "Soil (bottled feed)": moisture-based irrigation by VWC with long dry-downs, dosing caps halved, pH control on the water only, no reservoir pH-down below 6.0. Time-to-harm on pump failure: days. Automation level 2 [4].
Common mistakes
- Flushing before harvest. Pre-harvest flushing is a Myth that costs you yield and resin: it starves the plant while it packs on its final weight, trichomes and resin. Feed through to harvest at stage strength. The only valid flush is a root-zone flush to clear salt build-up, at any stage (see the flush debate).
- Small cups watered daily "a little bit": the pot never dries and never fully wets.
- Hydro nutrient strength on top of a fertilised mix.
- Chasing every leaf spot by changing nutrients before checking runoff pH and watering rhythm.
Evidence
Container-media physics is Established [1]. Feed ranges for mineral-fed soil are Grower practice; for organic liquid feed in coir-based organic substrates, ~389 mg N/L maximised veg yield in one study [2], which shows how much higher organic N rates can run than people expect, but it is not a mineral-soil recipe.
