Nutrient-film bench for lettuce
Eight covered channels 6 m long on a bench falling 2 %, a pump in a covered tank, and a film of solution 1–2 mm deep: lettuce without soil in 30–50 days.
Build
Tier 5 · Second industrial revolution
Food
Moderate — the build is simple; keeping EC, pH and the flow right every day is the work
Two or three days for the bench, channels and plumbing
Crew 1–2
Home: Protected growing, Level 5 · Current, level 2 of 3 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A nutrient-film channel grows a crop with no substrate and very little solution, which saves water and fertilizer, but it has almost no buffer against a stopped pump or a power cut (soilless growing). The numbers that keep it working are FAO's: a fall of 1.2–3.0 % with no dips, a flat channel floor, 1–3 L/min into each channel, channels no longer than 12–16 m, black-on-white cover film, and a tank big enough to hold all the solution when the pump stops (FAO, Good Agricultural Practices for greenhouse vegetable crops, 2013). The bench is sized for lettuce, the crop NFT suits best, at Cornell's harvest density of 38 plants per m².
The bench in elevation
Fig 1
TimberWaterVoid / air
The bench in cross-section
Fig 2
TimberLoose fill / grainWaterVoid / air
Keyed parts
| 1 | Bench | Timber or steel frame 6 m long and 1.6 m wide, its top falling 12 cm, 2 %, from the feed end to the drain end, with no dips. |
| 2 | Channels | Eight flat-bottomed plastic channels, 10 cm wide, 20 cm apart: polyethylene, PVC or polypropylene. |
| 3 | Cover film | Black-on-white polyethylene 0.15–0.25 mm thick, white side out against heat, black side in against algae; a hole every 13 cm. |
| 4 | Feed manifold | Pipe across the high end with a small valve and outlet into each channel, set to about 1 L/min. |
| 5 | Catchment pipe | A large pipe across the low end that takes every channel's outflow back to the tank. |
| 6 | Tank | Covered and opaque, 200 L, under the low end: holds all the solution in the system when the pump stops, and buffers temperature and EC. |
| 7 | Pump | Submersible, at least 0.5 m³/h at the lift to the manifold, on residual-current protection. |
| 8 | Plant cubes | Lettuce raised in rock wool cubes or plugs, set into the holes once their roots have formed. |
Specifications
| Bench | 6 × 1.6 m, 9.6 m² | Channels may be up to 12–16 m long before the far plants go short of nitrogen and potassium. |
| Fall | 12 cm over 6 m, 2 % | FAO's range is 1.2–3.0 %; no dips, which hold standing solution and starve roots of oxygen. |
| Channels | 8, flat-bottomed, 10 cm wide, 20 cm apart | A flat floor keeps the stream 1–2 mm deep. |
| Flow | about 1 L/min per channel, 8 L/min in all | FAO: 1–3 L/min per channel, the lower end for lettuce. |
| Plants | about 365, 13 cm apart in the channel | About 38 per m², Cornell's lettuce density at harvest. |
| Tank | 200 L, covered | Must hold all the solution in the system when the pump stops; a larger volume changes more slowly. |
| Solution for lettuce | EC 1.2–1.8 mS/cm, pH 5.5–6.0 | Oklahoma State University HLA-6722; Cornell runs EC 1.2 above the source water and pH 5.8. |
| Crop time | 30–50 days | From setting out to harvest (FAO). |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Timber or steel for the bench | 1 frame, 6 × 1.6 m | Legs at least every 3 m; stiff enough not to sag, because a sag is a dip. |
| Flat-bottomed plastic channels, 10 cm wide | 8 × 6 m | Food-grade polyethylene, PVC or polypropylene. |
| Black-on-white polyethylene film, 0.15–0.25 mm | about 12 m² | Enough to wrap each channel's top with an overlap. |
| Covered opaque tank | 200 L | Light in the tank grows algae. |
| Submersible pump | 1, at least 0.5 m³/h at the lift | On a residual-current device; a spare pump kept ready. |
| Pipe and fittings for the manifold, with 8 small valves | about 4 m | One outlet and valve per channel. |
| Catchment pipe, large diameter, with end caps | about 2 m | Slotted or drilled to take each channel's end. |
| Rock wool cubes or plugs | about 365 per crop | Seedlings raised in them in a nursery tray. |
| Two-part fertilizer stock solutions | for 200 L at the target EC | Calcium kept in the second stock, apart from sulphates (Wageningen). |
| Acid and alkali for pH | small stocks | Nitric acid to lower pH, potassium hydroxide to raise it (Cornell); handle with gloves and goggles. |
Tools: EC meter and pH meter, with calibration solutions · Spirit level and a long straightedge, or a water level · Measuring jug and watch for the flow check · Hole punch or knife for the film · Thermometer · Gloves and goggles for the acid and alkali
Construction
Test the water first
Have the source water analysed for EC, pH, sodium, chloride, bicarbonate, calcium, magnesium, iron and boron before building. Where bicarbonate is 3–7 mmol/L, plan to neutralise it with acid to about 0.5 mmol/L; where sodium is high, use rainwater or other low-sodium water, because a closed system concentrates it (soilless growing).
Build the bench to a true fall
Set up the frame 6 m long and 1.6 m wide, with the top falling 12 cm from the feed end to the drain end: 2 %. Check it with a straightedge every metre: no dips and no humps, because a dip holds a pond of still solution and the roots in it go short of oxygen (FAO).
Lay and cover the channels
Lay the 8 channels 20 cm apart, centre to centre, flat floor down. Cover each with black-on-white film, white side out, and cut a hole every 13 cm along the centre line for the plant cubes.
Plumb the tank, pump, manifold and return
Set the covered tank under the low end, with the catchment pipe across the channel ends draining into it. Run the pump's riser to the manifold across the high end, with a small valve and outlet into each channel. Put the pump on residual-current protection, keep all cables off the floor, and site the tank's mains connection above any possible flood (electricity guide).
Fill, set the flow and test the shut-down
Fill the tank with water only and run the pump. With a jug and a watch, set each channel's valve to about 1 L/min. Then switch the pump off and watch the tank: it must take everything that drains back without overflowing. If the pump will run only in daylight, this test decides the tank size (FAO).
Make up the solution
Mix the two stock solutions into the tank to EC 1.2–1.8 mS/cm and adjust to pH 5.5–6.0, after calibrating both meters against standard solutions. Run it for a day and recheck.
Set out the plants
Raise lettuce in rock wool cubes or plugs and set them in the holes only when a good root system has formed (FAO), so the roots reach the film at once.
Run it every day
Every day: read EC, pH and the solution temperature, top up with water or stock, and correct pH; check that every channel is flowing and no plant is wilting. Renew the solution when sodium and chloride build up or growth slows. Harvest at 30–50 days; clean the channels and film before the next crop. A root disease spreads to every plant that shares the solution, so treat the return with a slow sand filter or UV, or renew the solution after any outbreak (soilless growing).
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Fall | 1.2–3.0 %; 2 % here, with no dips | Too little fall, or a dip, leaves standing solution and starves the roots of oxygen. |
| Flow | about 1 L/min per channel for lettuce, inside FAO's 1–3 L/min | Too fast and the stream gets deep and the roots short of oxygen; too slow and the far plants go short of nutrients. |
| Channel length | no more than 12–16 m | The far plants get solution already stripped of nitrogen and potassium. |
| Tank | takes all the drain-back with the pump off | Otherwise it overflows every time the pump stops, and the channels run dry when it restarts. |
| Solution for lettuce | EC 1.2–1.8 mS/cm, pH 5.5–6.0 | Outside these the crop grows slowly or shows deficiencies. |
How this fails
The pump stops — a power cut, a blocked intake — and with almost no solution in the channels the roots dry and the crop wilts. Keep a spare pump and an alarm.
A dip in a channel: standing solution, roots without oxygen, a row that grows slowly or rots.
A tank too small to take the drain-back: it overflows when the pump stops.
Light reaching the solution through clear film or an open tank: algae in every channel.
EC or pH not read for days: a deficiency or a burn across the whole bench at once.
A root disease in a closed system, carried to every plant by the shared solution.
Water and electricity together without residual-current protection.
How you know it worked
VerificationEvery channel runs at about 1 L/min with no standing water anywhere along it, the tank takes all the drain-back with the pump off, EC and pH have been inside the lettuce ranges at every daily reading, and the crop is ready in 30–50 days without wilting.
Scaling and variants
Longer channels, up to 12–16 m, keep the same fall and flow per channel. Fruiting crops need higher flows, up to 3 L/min per channel, and bigger channels, but most are grown in substrate instead. Warm climates, where a small volume of solution heats quickly along the channels, suit deep-water culture better, with 50–150 mm of solution (FAO). A larger greenhouse system follows the climate and oxygen rules in soilless growing and the modern greenhouse.