Variations on the Aquaponics Theme The basic recirculating aquaponics system is unquestionably a useful means by which to produce fish and plants. It is, however, not the only way that the production of fish and plants can be integrated and, in some circumstances, it may not even be the best way. In this section, we look at some other growing system options. Fish Tank Water and Soil-Based Gardening Are you a little bit excited by the thought of raising your own fish….but less so at the thought of having to use hydroponic systems to grow your vegetables and herbs? Do you struggle with the idea of having to buy/build a network of grow beds? Are you reluctant to abandon a tried and tested soil-based gardening system that has worked well for you? Maybe you’re keen to try aquaponics but you want to keep your soil-based garden, too. Have you considered integrating an aquaponics system into your soil-based gardening regime? As good as they are, the hydroponic growing systems that are most often used in aquaponics are not suited to every type of plant. Growing root crops, potatoes, sweetcorn and many large plants (like zucchini, squash, pumpkins and melons) usually favour other non-hydroponic gardening options. The cost of media-based grow beds may also be an issue for some people. Consider this! Given that you have to water your plants regardless of the type of garden that you choose, why not water them with water that the fish have used first. The immediate benefit is that the water that comes from your fish tank is already loaded up with nitrates (plant food). Plants will prefer it to “fresh” mains water because it doesn’t have chlorine in it. You are effectively producing a crop of fish for the same amount of water that you would have needed for your plants anyway……and, depending on your choice of growing system, you may still be able to largely avoid the need for purchased chemical fertilisers. The principal benefit of recirculating aquaponics is arguably its efficient use of water. People are attracted to the idea that the only water that is used in many aquaponics systems is that consumed by the plants or the small amount lost to evaporation. The Case for Water Replacement The truth is that water replacement is actually good for an aquaponics system. 2 Over time, nitrate and phosphate levels may exceed the needs of the plants. While not toxic in the same way as ammonia or nitrite, nitrate can be converted back to nitrite in certain circumstances and they may also contribute to off flavours (the muddy taste that some people complain about in freshwater fish). Dead bacteria accumulate in an aquaponics system. While some proprietary fish pellets discolour the water in an aquaponics system, the other explanation for tea-coloured water is the presence of dead bacteria. The biofilm that form on all of the surfaces that are exposed to water in an aquaponics system are the accumulation of bacteria - dead and alive. The bottom line with dead bacteria is that they do nothing useful in an AP system and they do contribute to clogging pipework - so minimising their numbers is a useful thing to do. The main case for water replacement has to do with solids. Many particulates are too small to be removed by conventional particulate filters but still produce ammonia, consume oxygen and may irritate the fish' gills. Periodic water exchange will, in conjunction with other mechanical and biological filtration processes, contain solids levels within acceptable limits. In any case, the real issue (in terms of water efficiency) is not that you replace water but rather what you do with the water that you change out. If you use the water that you change out for soil-based plant production (particularly if the growing system is water-wise) then there is absolutely no waste issue. Indeed, if you use the water for any purpose for which you would otherwise have used fresh water, then it is a useful thing. We regularly drain off modest quantities of fish tank water which we store in a 200 litre blue plastic drum. While the water is in the blue drum, we are able to add any additional ingredients (like chelated iron) or even a little compost tea. At the same time, we top the fish tank up with fresh rainwater which keeps the water quality parameters within appropriate limits. This integrated arrangement leverages our use of the rainwater and provides us the means to grow a wider range of plants. While virtually any soil-based growing system lends itself to partnering with an aquaponics system, my favourites include: . Square foot gardens . Raised sheet mulch beds . Wicking worm beds Raised Sheet Mulch Beds We first learned about sheet mulch gardening from the Permaculture books written by Bill Mollison and David Holmgren. The benefits of sheet mulch gardening include: . There is no need to dig the soil so it is perfect for lazy gardeners or those with physical limitations. It suppresses weed growth . It uses much less water than a conventional garden. The Online Urban Aquaponics Manual www.urbanaquaponics.com.au 3 . Large gardens can be established with modest effort. Inexperienced gardeners can achieve good results with little more than suitable mulch and a seasonal planting guide. Creating a sheet mulch bed is simplicity itself: . Choose a site which receives adequate sunlight and preferably one which is bordered by buildings, concrete paths or fences. This makes it easier to contain the garden. Raised beds are ideal since they resist weeds, provide excellent drainage and are easy on the back. Slash any coarse weeds. Don’t worry about removing them – simply lay them flat. Spread blood and bone, food wastes or well-rotted animal manure to kick start the biological processes that will result in fertile soil which drains well but which also retains adequate moisture. Cover the bed area with wet sheets of newspaper or cardboard ensuring that you leave no spaces. This hastens the decay of the existing weeds and prevents future weed growth. Spread clean mulch like seaweed, rice husks, sugar cane mulch, dry leaves, straw sawdust or wood shavings to a depth of at least 150mm. Avoid using hay which often contains weeds. If you use wood shavings or sawdust, ensure that you don’t bury them as they will deplete the soil of nitrogen. Soak the bed thoroughly and leave to drain for a couple of hours. To plant the garden, part the mulch and use an old knife to cut a slit in the paper. Place a handful of clean soil or compost (or worm castings) into the slit and plant a seedling into the soil. Carefully, draw the mulch back around base of the plant while leaving the leaves exposed. Repeat this as often as you have seedlings that can be accommodated in the bed. It’s important to fill the bed up with plants. Diversity is a cornerstone of this system so ensure that you plant different vegetables, herbs and flowers. Keep the bed well-watered during the first few months to encourage the microbes which will transform the soil. Over time, earthworm populations will increase and soil aeration will improve. In the second and subsequent years, the bed will also require less water as the retentive capacity of the soil is enhanced. If weeds appear, part the mulch and lay a sheet of newspaper over the weeds and cover well with more mulch. It’s important not to let them get a hold. You should bury vegetable and fruit scraps and other garden residues in the bed to provide plant nutrients. Over time, many self-sown seedlings will begin to appear. Avoid meat or fish scraps which will attract dogs, cats, flies and vermin to your garden. Worm castings are an excellent soil conditioner and plant nutrient source. Maintain the layer of mulch - we use sugar cane mulch and pea straw. It adds valuable nutrients as it rots down, it keeps your fruit and vegetables clean and it reduces water consumption. Our sheet mulch gardens are in four circular raised beds that are 600mm deep and 1500mm in diameter. We used sections of concrete storm water pipe. The benefits of these beds are that they are virtually indestructible and their height makes them what our computer-literate friends refer to as user-friendly. They resist weeds and they need much The Online Urban Aquaponics Manual www.urbanaquaponics.com.au 4 less water than a conventional garden. The concrete sections were half-filled with sandy loam topped off with a mix of nursery potting mix, coco peat and well-composted horse manure. We plant seedlings for vegetables with tiny seeds. We usually sow large seeds directly into the soil. To determine if the beds need water, we simply poke a finger into the mulch. If the mulch appears to be drying out, we water the beds. To summarise, sheet mulch gardening is an excellent growing system and, while it will grow anything, it is of particular interest to us because (given that we produce salad vegetables, leafy greens and soft herbs in our aquaponics systems) it is so well suited to the production of root crops and potatoes. Square Foot Gardening The brainchild of American Mel Bartholomew, square foot gardening is an excellent plant growing system that makes very efficient use of water, space and time. It is particularly useful for people who long to grow their own clean, fresh vegetables but who lack the skills or experience to garden in the conventional way.