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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 ≤ 10^o^C (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.
Calcium: Calcium is a limiting nutrient in aquaponics. Deficiencies will appear on new plant growth, as it is a mobile nutrient. Signs are small, deformed leaves that may exhibit scorched margins (tip burn) (Figure 18d). End blossom rot on tomato fruits is a characteristic sign of a Ca deficiency (Figure 18e). Even when adequate
Ca is present, it is restricted from entering the plant in humid conditions and has antagonistic relationships with potassium (Somerville et al. 2014). In addition to CaCO~3~, crushed coral can be used to maintain Ca levels and increase alkalinity in aquaponic systems. Using crushed coral is anecdotal but has been effective in small and medium sized system. Using a source that is sanitized is critical as to not introduce foreign organisms or disease into the system
Iron: Iron is one of the more easily recognized deficiencies. Fe deficiency is characterized by chlorosis (yellowing) between the veins of the leaf (Figure 18f). The veins themselves will remain green. As Fe is an immobile nutrient, symptoms will appear on new leaves. Signs appear similar to a Mg deficiency but are easily differentiated, as Mg symptoms appear on older leaves (Mg is a mobile nutrient). Chelated Fe is added to the system to maintain Fe levels at 2 mg/L.
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.