WHAT IS HYDROPONICS?
A great way of growing plants intensively and sustainably.
Hydroponics refers to any method of growing plants without the use of soil. Instead roots of plants are suspended in a sterile medium and the plants are fed by means of water containing nutrients. In many set-ups this nutrient solution is flushed over the roots periodically. This allows the roots to be exposed to oxygen in air in between each watering. In other systems the top part of the roots are exposed to air and the bottom part is submerged in water. The medium used is often referred to as culture e.g. sand culture refers to hydroponic plants grown in beds of sand. Other media include gravel, vermiculite, rockwool and water culture.
Although there are many different ways to set up hydroponics systems, not all are conducive to vertical crop production. Of those that are more favourable, NFT and aeroponics would be the best choices.
NFT Systems
Nutrient Film Technique (NFT) is a method of soil less culture where plants are grown bare rooted in long narrow channels which enclose a shallow stream of nutrient solution. The nutrient solution flows continuously through the channel (being recycled from the bottom to the top end by a pump). The plant roots grow a dense mat in the channel and the foliage sits on top, sometimes provided with support by a trellis system.
The channel is normally made from plastic (rigid or flexible polythene or PVC). The plants roots grow in the channel, and the top is usually supported by a wire mesh or trellis framework.
N.F.T. systems do not use a growing medium. Instead the plants are supported by a basket and the roots dangle in the nutrient solution. There is a constant flow of nutrient solution which means a timer is not required for the submersible pump. The nutrient solution is pumped into the growing tray using a tube it flows over the roots of the plants, and then drains back into the reservoir. This is an inexpensive method as it does not require the expense of replacing the growing medium for each successive crop. The film of nutrient must flow continuously. Roots dry out rapidly during power outages or equipment failure when the supply nutrient solution is interrupted. If it is being pumped by an electric motor, there should be a backup facility to cover power failure. NFT is perhaps the most popular hydroponic system used in the UK.
The key requirements in NFT are:
- To have a gradient which water flows along which will achieve an even flow - not too steep or low gradient and with no depressions or uneven spots.
- Inlet flow rate must be controlled: not too fast or slow.
- Width of channels where flow occurs must be wide enough to avoid damming up of solution by the root mat.
- Base of channel must be flat to ensure even depth of solution across the width.
The channels are made of water tight material such as PVC or plastic film. They are laid on a slight slope to allow the nutrient solution to flow from top to bottom (supported by a rack or on a bench).
The channel is enclosed as much as possible, with openings only needed for the top growth of the plants to come through. Even though the majority of space inside the channel is air space, being enclosed creates a very high level of humidity which roots can still grow in (in fact, this ensures adequate levels of oxygen in the root zone, which can sometimes be a problem when plants are grown in soil).
The main advantage with this type of system is that no growing medium is required. Instead the crops are suspended with their roots dangling into the channel. While NFT can be used to grow a wide range of crop plants it suits crops such as lettuce, herbs and salad greens which have smaller root systems which are less likely to cause blockages inside the restricted NFT channels.
In the past, NFT was used extensively for vine crops like tomatoes, peppers, and cucumbers, but there has been a switch towards the use of rockwool and perlite for these crops since in NFT culture the plant roots are deprived of oxygen during fruiting, and they die off. Nowadays various modified NFT systems incorporating rockwool or perlite are used for growing vine crops. Others have designed systems which have two or more tiers of NFT troughs using channels with rows of plantings two crops wide, and using A-frames to extend planting upwards.
Alternative Layouts for NFT
1. A Horizontal Rectangular Plane
This is the most common NFT system. The area being used has a slope both along and across the plane. The slope on the land should be graded evenly to achieve a smooth slope.
A catchment trench is dug along the low side of the area, and a watertight lining is put into the trench. NFT channels are laid across the slope at right angles to the trench. Nutrient solution is pumped into top end of the channels and allowed to trickle to the bottom end where it flows out into the catchment trench for recycling.
2. Vertical Tubes
EXAMPLE: PVC pipes hanging vertically from a framework.
Nutrient solution is sprayed into the top of the pipes where it creates a film.
The film of nutrient solution runs down the inside of the pipe.
Holes cut at intervals down the pipe have plants inserted into them.
3. Tier System
This involves a series of tubes mounted one above another, frequently on a wall. he nutrient solution is pumped to the top from where it is fed simultaneously into the ends of each of the tubes at the high ends of those tubes.
Solution trickles down the tube, is drained off at the low end and recycled into a storage tank from where it is pumped back to the top of the system.
NFT System Choices
Header Tank or Direct Pumping
The nutrient solution is normally stored in a sump or catchment tank which collects the run off from NFT channels. The nutrient solution can be delivered from the sump to the top of each channel one of two ways:
1. Directly where it is pumped into feeder tubes which the top of each channel.
OR
2. Via a Header Tank where it is pumped into a secondary tank (i.e. a header tank, which is placed at a level higher than the channels). The nutrient solution then flows via a system of delivery tubes into the channels by gravity.
The advantage of a header tank is that it will continue to operate for some time after the pump ceases to operate (if there is some problem with the pump).
Construction Materials
Some materials can have a phytotoxic effect in that chemicals in the material dissolve into the flowing solution and deter plant growth.
Gullies or channels have been successfully constructed from the following materials:
1. PVC house guttering.
2. Square PVC plumber’s down-pipe with holes cut in the top for plants.
3. Circular PVC drainage pipe with holes cut in the top for plants.
4. Corrugated fibre-glass sheeting with an overlay of flat plastic.
5. Asphalt-coated wood.
6. Folded polythene film pinned at the top to make a tube.
7. Concrete formed into gullies on the surface of the ground.
Solution Delivery
Normally solution is delivered as a continuous flow; but sometimes it is only delivered for a few minutes then stopped for 15 to 60 minutes before flowing again. In the case of plants which require very good aeration, this intermittent supply may be used to increase aeration in the root zone (particularly critical in established plants where the root system can clog the channel and slow down water supply).
Capillary Matting
No matter how carefully constructed the base of an NFT channel is, you will still almost always get some fall from one side of the channel to the other. Capillary matting is sometimes placed on the bottom of the channel to even out the flow of solution. The solution is soaked up by the mat and spread right across the channel, preventing the solution from flowing on one side of the channel, leaving the other side dry.
The capillary mat material must be absorbent, physically stable and chemically inert. There have been cases of phytotoxicity reported through the use of inappropriate materials for capillary matting. Materials used successfully include cellulose fibre, rockwool and various artificial fibres.
Channel Width and Length
The width for a single row of plants should normally be between 10 cm and 15 cm. For a double row of plants, the channel should be 25 to 30 cm wide. Multi-channels, more than two rows wide, are also used at times (where there is no great problem with plants growing in close proximity to each other).
NFT channels should not be any longer than 15 metres, although shorter lengths have advantages in warmer climates. This is because plants in the upper parts of long channels will remove a lot of oxygen (effect on nutrient uptake is small in NFT systems) from the solution before it reaches plants in the lower sections. Dissolved oxygen (DO) levels in NFT systems have been recorded to fall from 7-8 ppm to 0 ppm along channels as short as 10 m containing a crop with high oxygen requirements such as cucumbers, zucchinis, melons, tomatoes and similar large plants. A lack of dissolved oxygen in the nutrient will cause wilting of the plants and yield losses, and is a particular problem under warmer growing conditions when the plants requirement for oxygen increases substantially but the nutrient holds less DO as the solution increases in temperature.
Slope
The depth of the nutrient solution in the channel should not normally be more than a few millimetres. The major part of the root mat should not be submerged in solution.
Normally 1:150 or greater slope is required to achieve proper flow of solution where capillary mat is used on a flat bottom. If the bottom of the channel is made using corrugated sheeting, a stiff capillary mat can be laid over the top allowing a free flow of solution along the channel below. If the bottom is flat though, the flow along the bottom will be slowed by the capillary mat, hence requiring a greater slope along the channel to achieve the same flow rate.
Where capillary mat is not used, the slope can be as little as 1:200 to 1:250. The rate at which solution is fed into the channels should be as great as is possible without increasing the depth of nutrient solution to more than a few millimetres in the channel. This can only be determined by trial and error for each situation.
Many growers now use shorter NFT channels with adjustable slope or slopes as high as 1:60 or 1:40. This allows them to maintain the flow of the nutrient solution at a sufficient rate and prevent ponding as the root mat develops and slows the flow of solution over time.
Temperature
Ideally a solution should be kept at between 18°C and 22°C for most plants. There is a danger of solution overheating inside channels if they receive too much direct sun in the early stages of a crop. As the crop develops though, it will shade the channel and this becomes less of a problem.
Aeroponics
Aeroponics systems are really a specialised form of water culture. The roots of plants must be placed inside a container which is dark, e.g. black plastic tubes or compartments, to restrict algal growth. But plants can be grown in any opaque material which stops the penetration of light e.g. a trough. The containers are suspended and the roots are misted with nutrient solution every few minutes so that they do not dry out.
They are expensive systems to establish and so are not used extensively on a commercial scale. However, due to the fact that they are relatively lightweight they can be used upwards with relative ease. In this type of arrangement the plants and containers are supported on A-frames.
Interestingly, a number of research programmes have been instigated by NASA using aeroponic designs in laboratories. These have been conducted to look into the suitability of aeroponics for establishing plants in enclosed space stations since systems are spread out both vertically and horizontally. As such, they make economical use of space in a greenhouse or urban farm (or in a space station).
These systems require a timer to trigger each misting. The timer is connected to a pump which pumps nutrient solution from a storage tank through irrigation pipes and into mister nozzles.
Gravel and Loose Culture
Gravel, sand and other media used to grow food crops in hydroponics are not particularly well suited to vertical growing system incorporating multiple grow beds against a wall because of the weight and the extra cost of pumping water upwards. However, it has been adapted in urban farming where grow beds or planter boxes are laid out on multi-layered scaffolding inside an industrial building. Several layers of lighter weight media like vermiculite or rockwool could also be set up on a few shelves inside greenhouses.
In normal situations where loose media are used these systems can easily grow climbing and trailing plants upwards onto trellis. A lot of crops such as tomatoes, and cucumbers, as well as cut flower crops, are grown hydroponically this way.
