Tilling the soil before you plant can be difficult, especially if the land is rocky or paved over. But don’t let that stop you from growing vegetables! Soil-less agriculture is an alternative that requires less physical effort and uses less space. Two good examples are hydroponics and aquaponics. But which one better suits you? That depends on your preference for dealing with soluble fertilizer or live fish.
Hydroponic systems may have been utilized thousands of years ago (think of the legendary Hanging Gardens of Babylon!). It may seem contradictory to grow plants without any soil, but actually it often works better than in-soil gardening. For plants to flourish, they need just two things: essential nutrients and water. Thus, if nutrients are present in the water and delivered to the roots, the plant has no need for soil.
Hydroponic systems don’t need arable land and consume fewer resources, yet crops can be higher quality than those grown by traditional methods. These benefits are increasing the popularity of hydroponics, which is spawning many inspiring, creative applications in urban gardening.
Aquaponic systems are another soil-less innovation, one that combines growing plants and raising fish. Fish excrete waste, beneficial microbes convert the waste into usable nutrients for plants, and the roots naturally filter the water to provide a clean living environment for fish and microbes. It’s a symbiotic relationship that results in an incredibly efficient system!
Hydroponics typically utilizes 6”-deep grow beds, since the roots can easily spread out within the aquatic solution without risking root compaction. Aquaponic grow beds must be deeper, a minimum of 12”, so fish have enough room to swim around.
Another difference is the environment. Hydroponic systems are very sterile, since there’s no need for extraneous growing media to support the plants or root systems. Aquaponic environments, on the other hand, must harbor the beneficial microorganisms around the roots.
Hydroponics is best for plants with high nutrient needs; you simply adapt the solution to meet the plant’s needs. Aquaponics typically supports plants with lower needs, such as lettuce, leafy greens, and herbs. Or, if you need more nutrients and the tank is big enough, you just add more fish!
Speaking of nutrients, don’t forget to feed the fish in an aquaponic system! How much and what feed depends on the fish you’re raising.
Acidity and Salt
Correct water acidity is essential to any aquatic-based growing system. The optimum pH in hydroponics is 5.5–6.5. Be aware that salt-based fertilizers, recirculated over and over in the nutrient solution, will naturally build up salt content, raising the electrical conductivity (EC) of water. Unchecked, it could reach levels high enough to damage the plants.
Aquaponic water should be neutral or slightly acidic, with an optimal 6.5–7.0 pH to safely harbor your fish. Fish waste has very little salt, so high EC is rarely a concern for plants. However, fish waste does add acid to the water, so monitor the pH level.
With aquaponics, you must feed your fish daily. However, other than checking the pH and ammonia levels weekly, there’s usually no need to flush and replace the nutrient solution, thanks to the naturally occurring symbiosis that keeps the levels in check.
With hydroponics, it’s necessary to periodically drain the aquatic solution and replenish with a new batch before the salts become concentrated. This means monitoring the pH, EC, total dissolved solids, and nutrient concentration.
As you can see, the systems vary in upfront labor vs. upkeep needed. Which one is better for you? Both are popular, and both provide the grower with distinct advantages over traditional gardening. Have fun trying both of them!
Amjad Ahmad, Cooperative Extension Service, Sustainable & Organic Agriculture Program, UH College of Tropical Agriculture & Human Resources