Fixing nutrient deficiencies in aquaponics plants

Fish feed supplies most of what plants need, and it supplies nitrogen especially well. It doesn't supply everything in the right proportions. The FAO manual on small-scale aquaponic food production names iron, potassium, and calcium as the nutrients aquaponics plants most often run short of, because fish need far less of them than plants do. Each one has a fish-safe fix, and the leaf symptoms are distinct enough to tell them apart once you know where to look.

Check pH and light before adding anything

A plant can show deficiency symptoms with plenty of a nutrient in the water if the pH keeps the roots from absorbing it. Iron is the classic case, since it becomes much less available as pH climbs above 7. Before adding a supplement, test pH with the method in our water testing guide. If it's sitting around 7.5 or higher, bringing it down gradually may clear the symptoms on its own.

Light is the other thing to rule out. Pale, stretched growth on a windowsill in winter is usually a light problem, and no supplement will fix it. Leafy greens and herbs grown indoors often need a grow light running 12 to 16 hours a day to grow well.

Reading the leaves

Where a symptom first shows up on the plant narrows down the cause. Plants can move some nutrients, including nitrogen, potassium, and magnesium, out of old leaves to feed new growth, so shortages of those show on the lower, older leaves first. Iron and calcium can't be moved once they're in place, so their shortages show on the newest growth at the top.

Iron

Iron deficiency shows as yellowing between the veins of young leaves while the veins themselves stay green, a pattern called interveinal chlorosis. Basil, leafy greens, and strawberries show it readily, and the FAO manual notes that many new systems hit it in their first two to three months. Aquaponics growers use chelated iron, where the iron is bound to a carrier molecule that keeps it dissolved. The manual recommends dosing to about 1 to 2 ppm of iron in new systems and again whenever symptoms appear. Fe-DTPA is the usual chelate for systems running near pH 7, while Fe-EDDHA stays available at higher pH. Follow the product label, since iron content varies between products, and add it in a spot with good water flow. Plain iron sulfate drops out of solution quickly at aquaponics pH and does little.

Potassium

Potassium shortage starts on older leaves as yellowing, then browning and curling along the edges and tips, as if the margins were scorched. Fruiting plants like tomatoes and peppers show it as weak flowering and small fruit. Fish feed carries relatively little potassium, so systems growing fruiting crops usually need more of it, and the simplest way to add it is through pH correction. When the slow downward pH drift described in our nitrogen cycle guide needs correcting, use potassium carbonate or potassium hydroxide as the buffer.

Potassium hydroxide is a strong base. Wear gloves and eye protection, dissolve a small amount in a cup of water first, and pour it in slowly where the water is moving well. Raise pH gradually, by a few tenths a day at most, and retest before adding more. Potassium carbonate is gentler and adds to the carbonate buffer at the same time.

Calcium

Calcium deficiency appears on the newest growth. Lettuce develops tip burn, where the edges of young inner leaves brown and die back, and tomatoes develop blossom-end rot, a dark sunken patch on the bottom of the fruit. Hard tap water often supplies enough calcium on its own. Where it doesn't, calcium hydroxide (hydrated lime) can take turns with the potassium buffer in your pH routine, and the FAO manual suggests leaning toward calcium for leafy greens and potassium for fruiting plants. Calcium carbonate, such as crushed oyster shell or cleaned eggshell in a mesh bag, dissolves slowly as pH falls and buffers the system as it goes. Tip burn in lettuce can also come from fast growth in warm, still air, so keep some airflow over the leaves.

Magnesium

Magnesium deficiency looks like iron deficiency on older leaves, with yellowing between the veins starting at the bottom of the plant. A small dose of Epsom salt (magnesium sulfate) corrects it. In a few gallons of water it's easy to overshoot, so dissolve a pinch first, add it, and watch the new growth for a week before adding more.

Nitrogen

General pale yellowing of the older leaves, with slow growth all round, usually means nitrogen is short. In aquaponics that's a question of fish and feed. If nitrate reads near zero, the plants have outgrown the fish load, and a slightly higher feeding rate or a few more fish will fix it, a balance our guide to feeding and stocking covers.

Products to keep out of the system

General hydroponic nutrients and garden fertilizers are built for systems without animals. Many include copper, zinc, and other trace metals that can harm fish, and shrimp and snails are especially sensitive to copper. Leaf-shine products, systemic plant feeds, and pest sprays are also risky, because whatever drips off the leaves ends up in the tank.

If you supplement beyond iron, potassium, calcium, and magnesium, use products labeled safe for aquaponics or aquariums. Change one thing at a time and wait a week or two to see the effect on new growth, since leaves that are already damaged won't recover their color.

Picking plants that need less help

Leafy greens and herbs have modest needs and do well on fish waste plus a little iron. Fruiting crops need more potassium and calcium and are harder to keep happy in a small system. If you'd rather supplement as little as possible, start with the species in our article on top plants for filtration efficiency, and add fruiting plants once the system has run steadily for a few months.