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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 CaCO₃, 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.

FIGURE 18d Calcium deficiency in lettuce

FIGURE 18e Calcium deficiency

FIGURE 18f Iron deficiency