Here we continue from Part 1 from the prior issue. URL to part 1 – https://www.pondtrademag.com/articles/ar-243/
Numbers of Fish
This can be quite a hot topic of debate amongst people who practice aquaponics. Stocking levels of fish within a system can be as high as many intensive recirculating aquaculture systems; however the higher the stocking density the higher the likelihood of things going wrong. In very heavy stocking densities you need to keep a constant eye on all water parameters to be sure that conditions are kept at the optimum.
If you lower the stocking levels of fish then you lower your levels of risk and stress. Growth rates of plants in lightly stocked systems can still be very impressive. This eight-bed system was stocked with only 70 fish. That’s less than 9 fish per growbed. The fish tank is 5000L and there’s a 1000L sump on the system. The fish in the system at the time of taking this photo were trout and they were around 300 – 400g. The plant growth in the eight beds was fantastic. A wide mixture of plants were grown in the beds.
We are often asked which plants grow well in an aquaponics system and it seems that most herbs and vegetables adapt well to aquaponics. Of course some plants won’t do as well when using different methods. Media filled beds seem to be the most successful for growing a large range of plants, and you can grow just about anything.
We often hear people say “But isn’t aquaponics only good for leafy green plants?” This is a fallacy that has been perpetuated for quite some time, but as you will see from the list at the bottom of this page, aquaponics will grow just about anything. This Habanero chili plant on the right grew a fantastic crop of super hot chilies, and a single tomato plant in a nearby grow bed produced well over 30 kg of tomatoes. Some other fruiting varieties that perform well are eggplant (aubergine), capsicum (bell pepper), bean, peas and many more.
What about root crops? No problem, although you are probably better off growing potatoes in the ground, they will still grow successfully in an aquaponic grow bed. Carrots are another great root crop in media beds. Carrots harvested from grow beds have no sand on them and although you might wonder how they grow in a bed filled with gravel they do a surprisingly good job.
Beetroots are another winner so we grow a lot of fresh beetroot and they grow to a fantastic size if you let them go long enough.
What about deficiencies?
Just as with all gardening you may get some deficiencies in your plants, but generally this can be dealt with very simply. Seaweed extracts are a great way to supplement almost all minerals that might be lacking in an aquaponic system. Seaweed extracts come in many different forms, and consideration towards harmful additives needs to be paramount as anything you add in the system will be passed on to both fish, bacteria, plants and you. You can also use powdered mineral substances. There are a number of different ones on the market, but once again, you must take care in their application if you haven’t used them before. Remember, it’s not just the plants you’re caring for but also the fish and the bacteria population. The best way to stay on top of deficiencies is to use a good quality aquaculture feed for your fish. Most good quality feeds have a percentage of ocean caught bi-product, this means that there’s a good range of minerals and trace elements within the fish feed. We have some systems that have not had any supplemental addition of minerals for well over 12 months and plant growth is exceptional.
Can I plant seeds?
Definitely. In media filled beds we generally use a combination of seedlings and seeds. When first planting a grow bed in a new system, we recommend sprinkling a mixture of seeds over the grow bed, as well as planting seedlings in the bed. Planting seedlings is simple. We recommend that you use normal seedlings but before you plant the seedlings, wash off the majority of the potting mix from the root ball in a small bucket of water. The potting mix should wash off fairly easily with just a gentle shake in the water.
While planting out the seedlings in the beds the media agitation causes the seeds that were sprinkled over the bed to fall down in between the media where it can absorb water and safely germinate. During planting trials I’ve found that this method has many benefits. As the seedlings grow and shelter the germinating seeds they tend to dominant the beds, and many of the germinating seeds can only grow very slowly. However, once the planted seedling matures and gets harvested, this opens up the canopy giving the small plants that germinated from seed a chance to take off. These plants now have a mature root system, and a great head start so they can grow very quickly. This mimics natural forest ecosystems where young trees and other undergrowth grow quite slowly, until a large tree dies and the canopy opens up allowing light to get to the lower plants, which then stimulates their growth.
What are growth rates like?
Growth rates of plants in aquaponic systems can be quite phenomenal. In fact a trial by Dr. Nick Savidov in Canada found that aquaponic growth rates can exceed hydroponic plant growth by up to four times with some vegetables and herbs. The advantage of aquaponics over soil grown vegetables is that during warm weather the plants get as much water as they need, due to the regular flooding of grow beds in an aquaponic system. Plants grown in the ground can use the water around their root system very quickly in hot weather, leading to wilting from a lack of water on a hot day. Plants in an aquaponic system get watered constantly, so they always have water, no matter how hot the weather is.
Here are some young plants in a grow bed. Joel Malcolm photographed these everyday to keep a record of the growth rates of the plants. The bed was planted with different varieties of basil, mizuna, cucumber and broccoli.
13 days after the first photograph, all of the plants are growing quite well.
25 days from the first picture. Joel has already harvested a couple of cucumbers and there are many more to come, with plenty of fresh herbs and salad greens also available.
Joel has grown chives that were over one metre (approx 3´ 4˝) in length in the same system as you see above.
Of course the plants you grow in your system may be limited by the type of aquaponics system you have, or more to the point, the type of plant growing system. If you have an NFT system you will not be growing carrots or large tomato plants because the small NFT pipes will not allow such growth. If designing your own aquaponic system you must put some thought into your plant growing method, and what you want to grow. This is one of the reasons why we like deep media filled beds at Backyard Aquaponics. When you have deep media beds there are no limitations on growing different plants.
This is a list of plants that grew well for members of the BYAP online discussion forum. This is by no means an exhaustive list of all plants that will grow well, just a few of the great performers that forum members have grown.
Simpson’s Curled (Lettuce)
Oregon Sugar Pod (Peas)
Bloomsdale Savoy (Spinach)
White Bunching (Onion)
Tomato (Grosse Lisse)
Silver Beet (Giant Fordhook)
Lettuce (All seasons)
Basil (Sweet green, purple, curly leaf and Thai)
Parsley (Flat leaf and curly leaf)
Numerous tomato varieties
Yugoslavian watercress (also known as Lebanese watercress or Bulgarian watercress)
Chives (normal and garlic variety)
Chilies, many varieties
Kohl Rabi (Purple vienna)
Eggplant (Black beauty)
Capsicum (Californian wonder, Yolo wonder, Long sweet yellow)
Cucumber (Burpless, Armenian)
Dwarf beans (Butter beans)