Introduction
More than 150 different vegetables, herbs, and flowers have been grown successfully in aquaponic systems. Plants suited to aquaponic systems are typically fast growing, have shallow root systems, and a low nutrient demand, such as leafy greens and herbs. Fruiting vegetables, such as tomatoes, cucumbers and peppers, also do well but they have higher nutrient demands and are more appropriate for established systems with adequate fish stocks. But there are some plants that don’t grow well, some that don’t make sense in terms of economics, and some that probably won’t work well due to space restrictions. Root crops, such as potatoes, sweet potatoes, turnips, onions, garlic, and carrots, typically do better in traditional culture, though they can be grown successfully in deep media beds (Somerville et al. 2014a).
There are some crops that require a larger investment, and if the intention is to grow a marketable crop for profit, then these crops are not cost-effective to grow. Radishes fall into this category, given their relatively low market price, as do some lettuces and leafy greens when their soil-grown counterparts are in season. However, there may well be niche markets that will pay higher-than- average prices for out-of-season vegetables, for crops not easily grown in the area, or for the novelty of hydroponically grown vegetables.
Aquaponic systems are finite spaces. This usually rules out growing fruit and nut trees, as well as most shrub-type plants, although bananas and papayas have been grown successfully at the Zurich University of Applied Sciences. Not only would the system require an enormous reservoir or tank to house the root system, but the amount of space needed to accommodate the plant itself would also need to be very large. Squashes and melons fall into this category, as can vine tomatoes which need trellising or some other structure for their cultivation. While there are hundreds of successful hydroponic operations growing tomatoes, these are typically in large greenhouse settings. Similarly, cucumbers can do reasonably well, but most heirloom species do not because they require a trellis system for their heavy fruits and many square metres of space per plant for their vines and foliage. Other vining crops that can outgrow their space and can be a nutrient drain include peas, pole beans, nasturtiums, and hops. While they all can be grown in a hydroculture system, they require a lot of work. The height of the grow lights must be regularly adjusted, the nutrient levels need to be adjusted according to the stage of plant growth, the trellising needs constant inspection and the provision of additional supports, and frequent pruning needs to be performed to successfully grow vining crops in a hydroponic set-up. On average, plants can be grown at the following density (Somerville et al. 2014b):
Leafy greens – 20-25 plants/m2
Fruiting vegetables – 4 plants/m2
These figures are only averages, and many variables exist depending on plant type, and therefore should only be used as a guideline.
When building a new farm, crop choice impacts sales, space, and technique. There are two types of cropping system: monocrop (or monoculture) is a system with a single plant type or variety; polycrop (or polyculture) is a system with different plant types and varieties. The choice between a variety of crops or a single plant type must be made with an eye on logistics, sales, experience, and pest control. The biggest advantage in favour of monocropping is simplicity. It can beat polycropping in terms of ease of sales, and is easier for new farmers when it comes to logistical overheads. If you’re growing a single crop, you’ll only ever need to prepare and ship your product in a single fashion. However, monocropping opens up the possibility of exhausting demand and, if combined with poor pest control, runs the risk of losing the entire yield at once. Polycropping gives farmers the possibility to meet a variety of demand, and is inherently more robust and resistant to pest outbreaks as there is a lower chance of the entire operation being compromised. However, members of the same family should be avoided, as these tend to be susceptible to the same bacterial, fungal and viral diseases, and to share common pests. Tomatoes, bell peppers and aubergines, for example, belong to the same family (Solanaceae), as do cabbages, pak choi, mustard greens and kale (Cruciferae or Brassicaceae). A crop set for polyculture requires crops with overlapping pH and temperature preferences.
Polycropping may also involve the use of companion plants. Companion planting is a small-scale intercropping method that is very common in organic and biodynamic horticulture, and is based on the observation that the association of different plants can have a mechanical, repellent or dissusasive effect against pests. The degree of success depends on the level of pest infestation, the crop density, the ratio between the crops and the beneficial plants, and the specific planting times. Companion planting can therefore be used in combination with other strategies within an integrated plant and pest management protocol (see Chapter 8) to obtain healthier plants in an aquaponic system (Somerville et al. 2014a). Some plants are also incompatible with others. For example, members of the cabbage family benefit from a number of companions, including aromatic herbs, spinach and herbs, but they are incompatible with strawberries and tomatoes.
Annual plant production rates in aquaponic systems vary depending on the species grown. Lettuce has been grown at different densities (16 to 44 plants/m2) and crop lengths (21-28 days), mainly on floating raft systems, resulting in yields ranging from 1.4 to 6.5 kg/m2. Basil is another widely tested crop, with densities of 8-36 plants/m2 producing yields of 1.4 to 4.4 kg/m2 for crop cycles of 28 days. Warm temperature crops have also proved to be very productive, such as water spinach which produced yields of 33-37 kg/m2 in 28 days at a density of 100 plants/m2, while okra produced yields of 2.5 and 2.8 kg/m2 in less than three months at densities of 2.7 and 4 plants/m2 respectively. Speciality and culinary herbs such as samphire (Salicornia) and saltwort (Salsola) gave yields of 7 kg m2 in 110 days and 5 kg m2 in 28 days respectively (Thorarinsdottir 2015).
Vegetables fall into three categories based on their overall nutrient demand. Low nutrient demand plants include leafy greens and herbs, such as lettuce, chard, rocket, basil, mint, parsley, coriander, chives, pak choi and watercress, and legumes such as peas and beans. At the other end of the spectrum are plants with high nutrient demand, sometimes referred to as ’nutrient hungry’. These include the botanical fruits, such as tomatoes, aubergines, cucumbers, courgettes, strawberries and peppers. Plants with medium nutrient demand are members of the cabbage family, such as kale, cauliflower, broccoli, and kohlrabi (Somerville et al. 2014a).
Aquaponic systems need to be balanced. The fish (and thus the fish feed) need to supply adequate nutrients for the plants, and the plants need to filter the water for the fish. Fruiting vegetables require about one-third more nutrients than leafy greens to support flower and fruit development (Somerville et al. 2014b):
Leafy greens – 40-50 g of fish feed/m2/day
Fruiting vegetables – 50-80 g of fish feed/m2/day
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).