NFT hydroponic system 60 plants, 2 m channels

You prepare the young plants separately, on an indoor seedling table, about 3 weeks ahead of transplanting. Several batches are therefore under way at the same time: those germinating on the seedling table, and those occupying the four channels at different stages. Write the transplanting date of each channel on a label fixed to its end, which is enough to tell you which one is ready to harvest the following week.

The profile measures 100 by 50 mm and the positions follow one another every 125 mm, which gives the density for short-cycle leaf crops: cut-and-come-again lettuces, batavias harvested young, lamb's lettuce, rocket, spinach, basil, parsley and coriander. Their root ball stays light and their fine roots do not clog the channel before harvest. If you are aiming for a head lettuce at full size, leave every second position free, since it needs about twenty centimetres between plants.

Plants that fill out over several months eventually occupy the full height of the profile, and their fine roots then start to slow the flow instead of following it. If your project is a long-term crop, such as strawberries or herbs grown as bushes, it is an 80 mm-high channel, spaced at 200 mm, that you should be looking at. Tomatoes and peppers, finally, need a real volume of water per plant, which only a deep water system can give them.

For the 64-litre reservoir, work out the dose by multiplying the per-litre dose by the volume. When the first true leaves appear, each of the three parts of TriPart is added at 1 ml per litre, which means 64 ml of each. In full growth, TriPart is added at 1.8 ml of Grow, 1.2 ml of Micro and 0.6 ml of Bloom per litre, which comes to roughly 115 ml, 77 ml and 38 ml for a full tank, giving a conductivity of 1.3 to 1.8 mS/cm. Conductivity reflects the amount of dissolved salts, and it tells you whether the solution is keeping pace with what the plants are consuming. For lettuces, then adjust the pH to between 5.6 and 6.0, once all three parts have dissolved.

When all 60 slots are occupied, the solution is depleted much faster than with a single planted channel, and it goes out of balance that much sooner. Use conductivity as your guide rather than the calendar: when it rises even though you have added nothing, the plants are taking up water faster than salts, and a top-up of clean water corrects the imbalance. Replace the whole solution every 2 to 3 weeks, more often when all four channels are full of mature plants.

Water from a fish tank can feed these channels, provided it first goes through mechanical filtration: a film a few millimetres deep clogs quickly, and 60 positions multiply the places where deposits can settle. You also give up the freedom to lower the pH, since the range that suits fish and nitrifying bacteria, around neutral, takes priority over the one the plants would prefer.

What you gain comes down to the growing area: the more plants there are, the more nitrogen they take up, and 60 planting spots help balance a pond whose fish produce a lot of waste. This nitrogen comes from the breakdown of waste by bacteria, as described in the nitrogen cycle. The volume of the pond then acts as a buffer, and the pH and temperature vary far more slowly than in a 64-litre reservoir.

You check the pH and conductivity in the reservoir, top up the level, adjust the nutrients, and make sure the water comes out properly at the end of the four channels. The usual guideline for nutrient film is a slope of about 1 cm per metre, which means 2 cm of fall between the two ends of a 2 m channel. A spirit level placed on each empty channel is enough to check it after moving or cleaning. While you are at it, rinse the pump strainer, since a partly blocked filter reduces the flow to the four outlets well before the film of water looks abnormal.

Twice a year, when a channel is empty, remove its lid and scrub the inside along its whole length. Over two metres, it is also a chance to check that the profile has not sagged in the middle: a dip of a few millimetres holds a puddle, and that standing water turns green. If you find one, an intermediate support under the low point solves the problem; if the flow remains uneven, the replacement channels in the same profile can be ordered individually.

You receive the four 2 m channels with their removable lids, each with 15 planting holes, and their factory-bonded end caps, the 23 mm square-tube supporting frame, the 64-litre reservoir, the Hailea HR1000 pump with a nominal flow rate of 1,050 L/h together with its supply line, the return circuit and 60 net pots of 5 cm. The lids lift off by hand, which gives access to the whole length of the channel for scrubbing.