Short answer
It depends on the medium. In coco, rockwool and hydro, feed every irrigation at stage strength. In bagged soil, feed every second or third watering at EC 0.8–1.6 mS/cm once the mix's own charge runs out. In living soil, water only and feed the soil by top-dressing. Grower practice, built on how each medium holds nutrients.
Why
Inert media hold almost no plant food, so the solution you pour in is the root zone's entire supply [1]. Plain-water irrigations in coco or rockwool dilute it, then the next feed spikes it again.
Potting soil is different. Peat, compost and the starter fertiliser hold nutrients on exchange sites and buffer what you add, so a feed every second or third watering keeps up without stacking salts.
More is not better. In deep-flow hydro, doubling feed strength from 2 to 4 mS/cm raised solution nutrients but not yield or quality [2]. The aim is steady supply, not maximum strength.
Runoff or pour-through EC is the check: it shows whether salts are building up or running short [3].
The numbers
| Medium |
Feed how often |
Feed EC (mS/cm) |
| Coco, rockwool, drip |
Every irrigation |
1.2–1.8 veg, 1.6–2.2 flower |
| DWC, RDWC, NFT |
Constant (reservoir) |
Same ranges |
| Bagged soil |
Every 2nd–3rd watering, after weeks 2–4 |
0.8–1.6 |
| Living soil |
Never; water only |
n/a (top-dress) |
What to do
- Identify your medium and pick the rhythm above.
- Measure feed EC including your water.
- Check runoff EC weekly in coco and monthly in soil.
- Runoff more than ~0.8–1.0 mS/cm above input and rising: feed weaker or less often.
- Runoff well below input: the plant wants more; raise strength slightly.
Common mistakes
- Plain-water irrigations in coco.
- Hydro-strength feeds every watering in fertilised soil.
- Feeding a seedling in fresh potting mix that already holds 2–4 weeks of food.
- Never measuring runoff, so salt build-up goes unseen.
Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Potting mix is a manufactured container medium, usually peat or coco with perlite, lime and starter fertiliser, designed to drain and hold air better than garden soil.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.Living soil is an organic growing medium managed to sustain an active soil food web, so microbes and invertebrates release nutrients and the soil is reused across cycles.Top dressing is spreading dry fertiliser or amendments on the soil surface, usually under mulch, so nutrients release into the root zone as the plant is watered.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.A standard way to sample root-zone pH and EC: water the pot to saturation, wait 30 minutes to 2 hours, then pour a set volume of distilled water on top and measure the first liquid that drains out.DWC (deep water culture) suspends a plant's roots in an aerated nutrient solution, typically in a bucket with a net pot lid and an air stone.RDWC (recirculating deep water culture) links several aerated buckets to a central control reservoir, pumping solution through all of them to keep conditions uniform.NFT (nutrient film technique) runs a thin, continuous film of nutrient solution along sloped channels, with roots resting in the stream and exposed to air above.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.
