Soilless growing: hydroponics and substrates

The main systems (FAO, 2013; Cornell):

• Nutrient film technique (NFT): bare roots lie in a sloping channel through which a thin film of solution flows and returns to a tank. Little substrate and little solution; best for short crops of 30–50 days — lettuce, herbs, leafy greens. Almost no buffer if anything stops.
• Deep water or floating rafts: plants in floating boards on a pond of solution, the oxygen supplied by air or pure oxygen. Holds a large, stable volume; the Cornell lettuce system.
• Substrate drip: each plant on a slab of rock wool or coir, fed by drippers. The usual system for tomato, cucumber and pepper.
• Ebb and flow: a bench or tray flooded and drained in turns; used for seedlings and transplants.

Raise transplants in plugs or rock wool cubes and move them into NFT channels only once a good root system has formed.

Channel design (FAO, 2013):

• Fall: 1.2–3.0 % from the feed end to the drain end, on a bench or a graded floor, with no dips: standing solution starves the roots of oxygen.
• Base: flat, not curved, so the stream stays 1–2 mm deep; the root mat's upper surface stays in the air.
• Flow: 1–3 litres a minute per channel (2–9 L/m²/h): lower for lettuce, higher for fruiting crops; about 2–4 L/m²/h for a young crop and 5–9 for a mature one.
• Length: no more than 12–16 m, or the plants at the far end get solution already stripped of nitrogen and potassium.
• Cover: black-on-white polyethylene film 0.15–0.25 mm thick, white side out against heating and black side in against algae.

If the pump runs only in daylight, the tank must hold all the solution in the system when it stops.

Roots need dissolved oxygen. In the Cornell lettuce ponds (2013):

• Below 4 mg/L growth is inhibited; below 3, signs of stress, and the crop can fail.
• About 7–8 mg/L is the working level (no advantage at 20), held with pure oxygen in recirculation; a small system can use an air pump and air stones, which reach a lower level.

Without added oxygen a pond falls to nearly 0 mg/L, and the plants die. Read it daily with an oxygen meter.

Have the water analysed before you start — EC, pH, sodium, chloride, bicarbonate, calcium, magnesium, iron and boron (Wageningen, Water quality and salinity aspects in hydroponic cultivation, 2016):

• Bicarbonate: many wells carry 3–7 mmol/L, which pushes pH up; neutralise it with acid down to about 0.5 mmol/L, keeping that small buffer.
• Sulphate: below 3 mmol/L.
• Sodium is taken up slowly, so it builds up at the roots in a recirculating system; use rainwater or other low-sodium water where you recirculate.
• Iron and manganese clog pipes and emitters: remove them by oxidation before the water enters the system.

One dS/m of conductivity is roughly 650 mg/L of dissolved salts where sodium chloride dominates.

Measure EC and pH every day, and calibrate both meters against standard solutions. Ranges by crop (Oklahoma State University fact sheet HLA-6722, Electrical Conductivity and pH Guide for Hydroponics), EC in mS/cm:

• Lettuce: EC 1.2–1.8, pH 5.5–6.0.
• Basil: 1.0–1.6, pH 5.5–6.0.
• Strawberry: 1.8–2.2, pH 5.5–6.0.
• Cucumber: 1.7–2.0, pH 6.0.
• Pepper: 0.8–1.8, pH 6.5.
• Tomato: 2.0–4.0, pH 6.0.

Cornell runs its lettuce at an EC 1.2 mS/cm above the source water's and pH 5.8, raised with potassium hydroxide and lowered with nitric acid. Make up two concentrated stock solutions, keeping calcium apart from sulphates, with which it precipitates (Wageningen).

Open systems drain the surplus away: FAO (2013) puts the leaching fraction — drained volume as a share of the water applied — at no more than 25–35 % in open soilless culture, enough to stop salt building up in the slab without wasting fertilizer.

Closed systems collect and reuse the drainage. Two problems follow (FAO):

• Ion build-up: the plants take up some nutrients faster than others, and sodium and chloride accumulate. Test the returning solution, and renew it when they build up.
• Disease: a root disease spreads through every plant that shares the solution. Treat the return with a slow sand filter or UV lamps before it goes back.

Lettuce in the Cornell system (2013) gets a daily light integral of 17 mol/m²/day from sun and lamps together, 24 °C by day and 18 °C at night, relative humidity 50–70 %, and CO₂ at 1,500 ppm when there is light to use it; the greenhouse step covers holding a greenhouse climate.

Water and electricity: every pump, heater and light on the electricity guide's residual-current protection, cables off the floor, and the tank's mains connection above any possible flood level.