Plants grown in an aquaponics system require the same light levels, nutrient amounts and water supply as when grown in soil. However, in contrast to traditional agriculture, aquaponics allows you more options for delivering each of the three essentials for growth. It’s also easier to control nutrient and water levels and adjust for oversupply without losing all of your crops right before harvest time.

Light

Each type of edible or ornamental plant has its own light requirements, usually broken down into the general categories of: full sun (6+ hours of bright and direct light a day), partial shade (4+ hours of bright light or 6+ hours of partial light), and full shade (less than 4+ hours of bright or partial sun a day).

Open air aquaponics systems are generally easy enough to orient to a southern exposure that provides 8+ hours of continuous full sun a day, barring any unforeseen shade producers like mature trees or nearby buildings. Supplying light in an enclosed greenhouse system is a little trickier. Even with the highest light transmission rate poly films or polycarbonate panels, some amount of the light is blocked. Greenhouse frames and solid ends can also cut off hours of early and late sun exposure.

Supplemental lighting is one option, but it comes with high upfront costs while also raising the electricity and maintenance bills. Choosing plants that need less light, or at least less intense levels of light, is a better option for most greenhouse growers. Complete indoor aquaponics systems will require a strong lighting system. Most full sun plants each need at least 20 to 30 watts, and this must be the actual in-use wattage, not just the equivalents used to advertise low-energy LED and CFL fixtures.

Nutrients

A big part of the appeal of aquaponics is the idea that the waste products released by fish can supply the nutrients required by the plants. Even with careful balancing and management, the best systems can still fall short of supplying all of a crop’s nutrient requirements. However, reducing the need for external fertilizers by just half or more can greatly increase the profit margins on the final harvested products. All plants, ornamental or edible, need some combination of the following three nutrients:

Nitrogen, or N. This is the most copious nutrient released from fish waste in the form of ammonia, but plants prefer to absorb it in a nitrate form. Nitrogen is required for strong, lush leaf and stem growth. Too much of it can kills plants from shock or at least delay flowering and fruit formation in favor of endless leafy growth. Leaf crops like lettuce and greens tend to require the most nitrogen.

Phosphorous, or P. Without phosphorous, plants grow weak and small roots, can’t extend their stems, and won’t set flowers or form fruit. Overall plant vitality, and resistance to pests and diseases, are based on phosphorous intake. It’s also a tricky nutrient to supply to plants in all forms of agriculture and horticulture. Fish waste provides a decent supply. Yet, plants that set a lot of individual fruits, like tomatoes, will likely benefit from some timely supplementation. Careful management is necessary for supplemental applications, as phosphorous is even more likely than nitrogen to set off algae blooms.

Potassium, or K. It’s often considered the least important of the three major plant nutrients, but some edible crops have a particularly high need for it. Potassium is used in most of the plant’s primary functions; including producing its own food through photosynthesis and new tissue growth. Without a steady supply, plants are sickly or just grow slowly in general. Some sources estimate that 90% of serious nutrient deficiencies in aquaponics are caused by low potassium levels. This occurs because proper potassium levels are necessary for the absorption of other nutrients like calcium. Although fish feeds tend to be high in potassium, little of that makes it through the nutrient cycle to the plants. Test regularly and don’t be afraid to supplement potassium levels, especially for heavy feeders like tomatoes and peppers.

NPK may be the macronutrients of the plant world, but they need more than these main three to stay healthy. Calcium, sulfur and magnesium in particular are the three most important trace nutrients for plant growth. Most well-balanced aquaponics systems supply plenty of all three to meet crop needs, but a lack of N, P or K can prevent the plants from absorbing these trace nutrients. Addressing an imbalance in NPK is often more valuable than trying to add supplemental calcium or magnesium, as the plants still won’t be able to absorb the extra nutrients.

Water

In general, there’s no need to be concerned about the amount of water supplied to the plants in an aquaponics system, unless there’s a clog or failure that dries out the trenches. As long as a steady flow is fed through the system, the plants will adapt to take up as much moisture as they need. This is one of the major benefits over traditional container or field agriculture.  In these more traditional methods, farmers must constantly calculate accurate irrigation rates based on evaporation, temperature, soil texture and other conditions.

Growing Medium

You’ll notice that the title of this section is not soil. Most edible and ornamental plants do need something in which to anchor their roots that’s a little more solid than water, although some species can grow entirely rooted in water. It’s just that there’s no need for this medium to be soil. Some of the most popular growing mediums for various types of aquaponics systems include:

• Fine crushed and sifted gravel; which was the original material used for the development of hydroponics, but eventually requires time-consuming cleaning to clear out accumulated biofilms.
• Coarse sand; which is similar to nutrient film techniques but doesn’t allow for recirculation of the water and is particularly prone to settling and clogging.
• Sawdust; which encourages good root growth and maintains a healthy structure through multiple uses but can be expensive now that it is in high demand for pressboard manufacturing.
• Rockwool; the most popular and commonly used substrate made from basalt rock liquefied and spun into fine strands, offering the most oxygen storage space and water holding capacity of all media.
• Coco coir, a fluffy fibrous material harvested from the outer layers of coconut shells, which is considered similar in features to rockwool while being biodegradable and most sustainable to produce.

Sawdust, gravel and sand tend to be available locally in more areas than coco coir and rockwool. Because you’ll need dozens of cubic yards to fill thousands of even the smallest net pots for a commercial system, it’s easier to find a local supplier for the materials than to order from a distant seller or over the internet. Rockwool and coco coir are usually compacted greatly for shipping, so you may be able to find an affordable deal on sourcing these materials from farther away without paying high freight shipping or special delivery costs.

Climate Considerations for Plant Choice

Just like selecting fish, it makes sense from a business standpoint to choose plants that already perform well in your general climate. Check out the USDA growing zone for the area where you plan to start your business. The zones are ranked from 1a to 13b based on winter and summer temperatures. These zones also help you determine when your first frost arrives in the fall and when spring temperatures are warm enough for frost-sensitive planting. Even if you plan to grow in a completely indoor or greenhouse enclosed, heated system, understanding the climate you’re dealing with is essential to determining how much heat you’ll need and when you’ll need it.