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A skill that is beneficial for aquaponics producers to keep in their toolbox is the ability to visually diagnose nutrient deficiencies. Once a plant exhibits symptom of a deficiency, severe stress is already occurring. Early detection and diagnosis are important.

Process of elimination can help growers successfully identify a nutrient deficiency. Key factors include recognizing where it occurs in the plant (mobile or immobile nutrient); taking note of the general appearance, such as color pattern or overall appearance; and eliminating other factors that may be causing the issue, such as light or heat damage. Below are common nutrient deficiencies that occur in aquaponics.

Nitrogen: Although not very common in aquaponic systems, nitrogen deficiencies most commonly occur when the fish culture units are undersized for the amount of plants in the system. Complete chlorosis (yellowing) of older leaves is the first sign and can spread to the whole plant if left untreated (Figure 18a). Other signs are slow or stunted growth and plants that look stretched. Nitrogen deficiency is typically not an issue in appropriately designed, well-cycled aquaponics systems.

Phosphorous: Phosphorous deficiency in plants is characterized by dark green and/or purple coloration in older leaves (Figure 18b). It may also manifest at the tips and edges of the leaves, giving them a burnt look. Availability of P to plant is greatly reduced when pH is outside the range of 6.0-7.5 and when temperatures are ≤ 10circC (Islam et al. 2019). Symptoms are more noticeable in young plants, which have a greater relative demand for P than mature plants.

Potassium: Potassium deficiency does not immediately result in visible symptoms. Leaf margins will appear tanned, scorched, and/or have small black spots that later aggregate into necrotic region (Figure 18c). Margins of the leaves will cup downward, and growth will be restricted. Potassium is a key nutrient for proper flower and fruit development. Inadequate supply of K will result in flowers' dropping off the plant. High K concentrations can reduce the uptake of Ca by the plant. K is a limiting nutrient in aquaponics and must be supplemented to maintain levels required for plant growth.