NFT (Nutrient Film Technique) was developed by Allen Cooper in England in the late 1960s. Today it is the standard in commercial greenhouses growing lettuce and herbs — if you buy a living lettuce in a pot, chances are high it grew in an NFT channel.
How the system works
The pump lifts solution to the high end of the channel, gravity carries it down over the bare roots, and at the low end it returns to the reservoir. The loop runs around the clock. The key word is film: the solution must be only a few millimetres deep. The lower part of the root mat sits in water and takes up nutrients, the upper part stays in humid air and breathes directly. Raise the level to 2 cm and you no longer have NFT — you have a poor version of DWC without aeration.
The numbers you have to hit
| Parameter | Value | Why exactly this |
|---|---|---|
| Channel slope | 1:30 to 1:40 (2.5–3.3 cm/m) | Less = puddles; more = the film races past |
| Flow per channel | about 1 L/min (0.5–2 L/min) | Below 0.5 the far plants starve, above 2 the film gets too deep |
| Film depth | 1–3 mm | Deeper floods the upper roots and suffocates them |
| Channel length | max 10–12 m, better under 8 m | Over longer runs the solution warms and is depleted |
| Channel width | 10–15 cm for lettuce and herbs | Narrower channels clog with roots faster |
| Solution temperature | 18–22 °C | Above 24 °C oxygen drops and Pythium takes off |
| Dissolved oxygen | above 6 mg/L | At 20 °C saturation sits around 9 mg/L |
| EC and pH for lettuce | EC 1.2–1.8 mS/cm, pH 5.5–6.2 | Standard for leafy greens |
| Reservoir | at least 2–4 L per plant | A smaller tank means swings in EC and temperature |
Why NFT delivers so much oxygen
A 2 mm film has an enormous contact area with air relative to its volume, so it re-saturates with oxygen continuously as it flows — no air pump involved. On top of that, most of the root mat is not in water at all but in saturated humid air above it. The result is speed: from transplant to a harvest-ready head of lettuce takes roughly 28 to 35 days, depending on light and variety.
Where NFT breaks down — honestly
- No reserve at all. There is no stored water in the root zone. A power cut on a warm day: the film is gone within minutes, the finest root hairs dry out in 15–30 minutes, permanent damage within 1–2 hours. DWC buys you hours, the Kratky method needs no power at all.
- The slope has to be exact. A 4 m channel sagging just 1 cm in the middle will have a puddle at one end and a dry spot at the other. There is no way around a spirit level.
- Roots build their own dam. With lettuce the root mat thickens after 5–6 weeks until it stops the flow. Behind the dam sits stagnant, oxygen-free water — a direct invitation to Pythium.
- Not for fruiting crops. Tomato, cucumber and pepper build root balls that completely block a 10 cm channel. They need DWC or more substrate.
- A gradient along the channel. The last plant receives solution that has already passed twenty others — warmer, lower in nitrogen, with shifted pH.
NFT forgives nothing. Either you hold the slope, the flow and the temperature, or the roots suffer — often before you notice anything is wrong.
NFT, DWC and Kratky compared
| NFT | DWC | Kratky | |
|---|---|---|---|
| Oxygen at the root | very high | high (with air pump) | medium |
| Buffer without power | 15–30 minutes | 6–12 hours | no pump, no problem |
| Solution per plant | least | plenty | plenty |
| For beginners | no | yes | yes |
| Fruiting vegetables | no | yes | limited |
| Checking | daily | 2–3× per week | weekly |
What a vertical tower borrows from NFT
A tower works on a related principle — the pump lifts solution to the top, it trickles over the roots and returns to the reservoir — but it fixes three NFT weaknesses. Every plant has its own pocket, so one vigorous root ball cannot dam the whole system. A 40 or 50 litre reservoir keeps the temperature steady and gives you hours instead of minutes during an outage. The solution travels 180 cm rather than 10 metres, so there is no nutrient gradient. The trade-off: a tower does not achieve the precisely thin film of a properly levelled NFT channel — negligible on a balcony, but for 500 m² of lettuce production NFT still wins.