NFT hydroponic system 28 plants, 1 m channels

On a system of this size, you work per litre, then multiply by 30. At the first roots, the three parts of the TriPart are each added at 0.5 ml per litre, that is 15 ml of each, and the conductivity of the solution, which reflects the quantity of dissolved salts, stays between 0.3 and 0.6 mS/cm. When the first true leaves appear, the dose goes up to 1 ml per litre, so 30 ml of each. In full growth, the Grow reaches 1.8 ml per litre, the Micro 1.2 and the Bloom 0.6, for a conductivity of 1.3 to 1.8 mS/cm.

Pour in each bottle separately, stirring, then adjust the pH last, once the three parts have dissolved. For lettuces, aim for a pH of 5.6 to 6.0, the range in which the nutrients remain available to the plants. Take a reading once a week on an established crop, and always in the reservoir, where the sample is representative of the whole circuit. Every 2 to 3 weeks, empty the reservoir and start again with a fresh solution, because the salts your plants do not consume build up in it with no visible sign. Between two changes, top up evaporated water with clean water.

Planting all 28 positions on the same day gives you 28 harvests in the same week, then an empty system for a month. Each channel holds 7 positions, and it is this unit that organises the routine: by transplanting one channel per week, after a month you have a channel ready to harvest every week, while the other three are at different stages. The reservoir solution is the same for all four channels, which works as long as the four channels hold species with similar needs.

This staggered approach means preparing your plants in advance, which can be done on an indoor seedling table without any complicated equipment. Allow about 3 weeks between sowing and transplanting, by which time a lettuce has formed enough roots to emerge from the bottom of the pot. The peat germination pods can then be slipped whole into the 5 cm baskets, with no need to remove them from the pot, which avoids breaking the roots just as they reach the thin film of water.

The positions are spaced 125 mm apart centre to centre, on a channel 50 mm high. This density suits crops whose root ball stays light and whose cycle is short: cut-and-come-again lettuces, lamb's lettuce, rocket, spinach, basil, parsley, chives and coriander. A head lettuce grown to full size needs about twenty centimetres between plants, so you will either harvest it younger or leave every second position free to bring it to a full head.

Plants that stay in place for several months, such as strawberries or herbs grown as bushes, gradually fill the height of the profile, and their fine root hairs eventually slow the flow. If you are aiming for these longer-term crops, the 80 mm-high channel system offers more headroom under the lid and a 200 mm spacing between centres. As for tomatoes and peppers, their root volume calls for a network of deep-water pots, where each plant has its own volume of water.

Feeding these channels with water from a fish tank is a common practice, and it changes two things. The first is mechanical: a film only a few millimetres deep clogs quickly, and the suspended solids in pond water call for mechanical filtration before the channels. The second is chemical, since the pH must then suit the fish and the nitrifying bacteria first, around neutral, rather than the more acidic range that lettuces prefer. In return, you get water that already carries its own nitrogen, from the fish waste, which the bacteria convert through the nitrogen cycle.

Once the channels are connected to a pond, it is the volume of the pond that acts as a buffer, rather than the 30 litres of the reservoir: the pH and temperature vary much more slowly from one day to the next, which makes the system calmer to run. Clogging is then monitored at the high point of each channel, where the supply line comes out. A trickle that weakens on a single channel indicates a partly blocked outlet, and you clear it with clean water without stopping the other three.

Each week, you check the pH and conductivity, top up the reservoir level and make sure the water comes out at the end of the four channels. The usual guideline for nutrient film is a slope of about 1 cm per metre, that is, 1 cm of fall between the two ends of a channel in this system. A channel that has sagged by a few millimetres in the middle holds a puddle, and this stagnant water turns green and loses oxygen around the roots sitting in it: a spirit level placed on the empty channel is enough to spot it.

Twice a year, or whenever you change crop, remove the removable lids and scrub the inside of the channels once the last plant has been harvested. The slimy deposit that lines the bottom, the biofilm, traps particles and root debris from one batch to the next. Then rinse your baskets and leave them to soak overnight before reusing them. Use this pause to rinse the pump strainer as well, since a partly blocked filter reduces the flow long before the stream looks abnormal.