Richard bray

Magnesium
Magnesium builds chlorophyll, which is the driving force behind photosynthesis. If there is not enough magnesium in the system, photosynthesis will suffer, and plants will not have enough energy to
Magnesium deficiencies result in green veins with yellow leaf tissue. Older growth will die off as the plant pulls magnesium from older growth to sustain new growth.
Agricultural lime will add magnesium as well as calcium, while simultaneously raising the pH of the water. Calcium toxicities are rare, so adding lime for a magnesium boost should not overload your system with calcium.
You can also use Epsom salt as a magnesium amendment, but you must be careful that you don’t add too much. Epsom salt is not salt, so it won't kill plants like salt will. However, too much can cause many issues. Use an average of 1 tablespoon per 1,000 gallons to correct magnesium or sulfur deficiencies.
Sulfur
Calcium, magnesium, and sulfur are difficult to separate. They all serve different functions, but amending one will affect all three.
Sulfur deficiencies are rare. Sulfates tend to build up in an aquaponic system; so many times there is an abundance of available sulfur. This
86
nutrient is responsible for healthy, green growth, and a deficiency results in yellow, lanky new stems and leaves. This makes it easy to confuse with other nutrient deficiencies, like phosphorus or nitrogen.
In general, yellow growth is almost never the result of a sulfur deficiency. If you have treated for nitrogen, phosphorus, magnesium, and iron deficiencies, but still have yellow stems, you may be low on sulfur.
Iron
Iron is another important component of photosynthesis. It is also one of the most common deficiencies in aquaponics, aside from potassium and calcium. While technically a micronutrient, it is a common problem in aquaponic systems, so it is important for diagnosing stressed plants.
Iron deficiencies result in chlorosis, or yellow leaves. A simple nutrient test should help clarify which nutrient is causing chlorosis, because this is a common symptom of many deficiencies.
Once you have confirmed an iron deficiency, the best remedy is chelated iron, as it is readily available to plants.