Choosing Your Growing Systems Aquaponics is the integration of recirculating aquaculture and hydroponics......in all of its forms. In this section, we consider the range of hydroponic growing systems that are available for use with aquaponics systems. Overview The choice of growing system will be largely driven by your resources and personal preferences. Other factors that will impact the choice of a growing system include: . the type of plants to be grown . how ‘hands on’ you want to be . the cultural needs of the plants The thing to remember is that no single growing system is ideally suited to all plants, so you’ll probably end up with more than one type of system….and you do have a wide range of choices. Aquaponics is the integration of recirculating aquaculture and hydroponic growing systems so (theoretically) any hydroponic system can be used for aquaponics. In practice, however, some hydro systems are better suited to urban aquaponics than others. For the purposes of aquaponics, hydroponic systems include the following: . Media-based Grow Beds – where various types of media are used to support the plants. Nutrient Flow Systems - where the plants are given direct access to the nutrient flow. Examples include nutrient film technique (NFT), deep water culture (DWC) and raft. Other options – generally include smaller systems like satellite pots, the Tray system and Autopot. Media-based Grow Beds Grow beds are the most popular growing system in use by backyard aquaponicists. They usually comprise a large waterproof tray that contains media which supports the plants. They may be made of fibreglass, plastic, corrugated steel and wood or plywood (fitted with a suitable liner). Old bathtubs and plastic barrels are also frequently recycled for use as grow beds. Probably the person most responsible for the current interest in gravel grow beds was Missouri farmer (and aquaponics pioneer) Tom Speraneo. While he wasn’t the first to use media-based grow beds (hydroponicists had been using them for decades) and he wasn’t even the first to use them in aquaponics (UVI had tried and rejected them years earlier), he made them work for his purposes and he popularised their use. 2 Virtually any media used in hydroponics can be adapted for use in aquaponics. Your circumstances and preferences will drive your choice of media. Here are the main options: . Gravel – is cheap to purchase - anchors plant roots very well – is very heavy - is as hard on the hands as it is on the back - washing gravel will consume large amounts of water. Some gravel is not inert and may contribute to pH spikes or mineral imbalances. Light Expanded Clay Aggregate – much lighter than gravel but much more expensive – better than gravel to work with. Vermiculite – great water retention – works best with open loop (non-recirculating) systems where timers control the watering cycle. Coco Coir – good water retention – will discolour the fish tank water so best used with open loop systems – some brands may be calcium-deficient and/or may contain high levels of sodium — rinse before use. Spent coco coir can be recycled through other backyard food production systems such as poultry litter and worm bedding. Perlite – very light - will gradually break up and has the unhappy knack of jamming pumps – best used in open loop situations - probably my least favourite aquaponics media. Continuous flow or Flood and Drain? If you decide to use media-based grow beds, you have three options when it comes to the watering method: . flood and drain . continuous flow #1 (surface) . continuous flow #2 (sub-surface) Each method has its own advantages and some limitations. Flood and Drain This watering regime is also known in hydroponic circles as ebb and flow. With the flood and drain method, the movement of water is controlled by timers, float switches or auto-syphons. As the name suggests, flood and drain grow beds fill up and then empty - in a continuous cycle. The benefits of this system include excellent aeration and nitrification….and the plants have unrestricted access to water and nutrients. The issues with the flood and drain method include: . fluctuating water level in the fish tank . requirement for a larger sump tank . ongoing need for adjustments to timers, float switches or auto-syphons. Continuous Flow #1 (surface) This method features a PVC watering grid which consists of pipes with holes drilled at pre- The Online Urban Aquaponics Manual www.urbanaquaponics.com.au 3 determined intervals. Seedlings are planted adjacent to each hole so that they receive a constant flow of nutrient-rich water. Aeration is excellent and (depending on the design of the watering grid) nitrification can be quite good, too. The disadvantages of this method are that the water is exposed to sunlight as it flows from the watering grid (and that may create the conditions for algal blooms) and plenty of flow is required to ensure that adequate water issues from the holes in the grid. Continuous Flow #2 (sub-surface) With this option, the nutrient-rich water enters one end of the grow bed and exits at the other end. The water is maintained at a predetermined depth. Expanded clay pebbles are the preferred media for this option because of their capacity for hydration – the water wicks upwards to create a damp (but not wet) zone at the base of the plants. Eventually, the plant roots grow down into the water while the plant crown remains dry. This ensures that the plant gets all of the water and nutrients that it needs without becoming waterlogged. Sub-surface continuous watering has a number of benefits including: . simplicity and reliability – there are no control devices to malfunction or get out of adjustment . constant availability of water and nutrients . lower build cost – no need for timers, float switches or auto-syphons . smaller pumps can be used – lower energy consumption . water re-entering the fish tank contains fewer solids – the lower water velocity allows any solids to settle out and to be trapped by the media more effectively . reduced algae problems because the inflow to the grow bed is beneath the clay pebble media Prospective issues with sub-surface watering systems include: . lower nitrification rates . less aeration I resolve both issues by inserting a trickle bio-filter into the water column immediately before the fish tank. Not only does this ensure optimum nitrification and aeration, the solids loading is limited to dissolved solids. My Preference While I have plenty of experience with flood and drain systems (and I love watching auto-syphons kick in and out), I prefer sub-surface continuous flow watering for media-based grow beds. I design aquaponics systems for maximum productivity which requires capture and removal of solid wastes, environmental control and a strategic approach to grow bed management. The Online Urban Aquaponics Manual www.urbanaquaponics.com.au 4 My latest design, The Queenslander, features sedimentation and mineralisation devices - and a Moving Bed bio-filter, which optimises water quality. The design also includes at least one continuous flow (sub-surface) grow bed. The way the grow bed is set up and managed also influences water quality. I place 150mm of clay pebble media in the beds and set the water depth to about 40mm. This ensures that the clay pebbles get hydrated to the correct level for the plants. We plant this grow bed out to short cycle plants like lettuce, Asian greens and soft herbs. This allows us to harvest the entire bed before cleaning it – and this happens on a five to six week cycle. The depth of the media makes for quick and easy cleaning. As the water exits the grow bed, it drains into the sump. From there, some of the water is pumped back into the fish tank. The surplus flow is directed through the bio-filter where any aeration or nitrification shortfall is resolved. In my experience, there is no discernible difference between flood and drain or continuous flow when it comes to plant growth. Grow Bed Depth Optimum grow bed depth is often the subject of hot debate among aquaponicists. There are those who argue that grow beds should not be less than 300mm in depth but the reality is that they can be less than that, so long as the plants are adequately supported and (to ensure the ongoing health of the nitrifying bacteria) the media remains moist and at a consistent temperature between watering cycles. In practice, I have found that properly managed grow beds can be as little as 100mm and will work just as well as the deeper ones. In fact, there are some benefits to be gained from using shallow grow beds. Cleaning shallow grow beds is a much easier task than it is with 300mm ones…..and filling them with media is a much less expensive undertaking. A 150mm grow bed will require half of the media of a 300mm one…..at half the cost and half the weight. A shallow grow bed can be made of less robust materials because it doesn’t need to support the same weight. Some other thoughts on Grow Beds: . They can be used in place of (or in addition to) trickling bio-filters. Blue plastic drums can be cut in half (vertically or horizontally) for use as grow beds. Cut in half on the vertical plane, a drum becomes two large tubs that can be used for dwarf fruit trees and other large plants like zucchini. If you decide to use plastic grow beds, consider the weight of the media. Only the strongest plastic containers will support the use of gravel. Elevated grow beds will (because of their weight) require robust supports.