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Most plants can be described by five main structures: roots, stem, leaves, flowers, and fruits. Dissolved nutrients and water enter the plant via the roots through passive and active (requiring energy) transport. The xylem, located in the stem, is a one-way transport channel that moves water and minerals from the roots' hairs into the main body of the plant through capillary action. The stem is typically the primary support structure for leaves, buds, and other organs. The leaves are the powerhouse of the plant and use solar energy to convert carbon dioxide (CO~2~) and water into glucose (energy) and oxygen (the photosynthetic process). Glucose is transported to other parts of the plant via the phloem. Flowers and fruits are the reproductive organs of the plant. Flowers require fertilization to develop into fruits. This can be accomplished by wind, insects, birds, mammals, etc. In a greenhouse, flowers can be pollinated by fans' gently shaking the plant, causing it to release pollen grains, or manually using a Q-tip or soft paintbrush.
Like in soil, plants grown in aquaponics derive their nutrients and energy for growth and reproduction from photosynthesis, dissolved inorganic salts, and metabolites produced by bacteria and fungi. In aquaponics, the nutrients are derived from feeding fish. Plants have some ability to select the rate at which they absorb various ions. Just because the nutrient is provided in adequate quantities does not mean that the plant is absorbing it. Plants have 16 essential nutrients required for optimal health and growth (Table 8). Essential nutrients are those that cannot be synthesized by the organism and are classified by structural, macro-, and micro-, delineating how much is required by the plant. Nutrients can also be categorized by their mobility. Mobile nutrients are elements that can be transported throughout the plant (usually to new growth) as needed. Once immobile nutrients are deposited in the leaves (typically older or primary leaves), they are fixed and unable to be transported to other parts of the plant. This can benefit producers when signs of a nutrient deficiency are apparent by narrowing down the causative ions. Mobile nutrient deficiencies occur in older leaves whereas immobile nutrients occur in new growth of the plant. Three nutrients are not provided in adequate quantities in fish feed to support plant growth. These nutrients are calcium (Ca), potassium (K), and iron (Fe). As discussed above, CaCO~3~ and K~2~CO~3~ are used to both amend pH and provide essential nutrients. Iron is supplemented in the chelated form, which keeps it soluble in the water and prevents it from oxidizing in the system. Fe-DTPA is recommended because it is more stable at the pH suitable for aquaponics (6.0-7.5) and more cost effective than other forms.
Table 8: Sixteen essential nutrients required by plants for optimal health and growth. Circled elements represent limiting nutrients in aquaponics. Mobile nutrients are represented by (m) and immobile nutrients by (i).
Essential Elements for Plant GrowthStructuralMacroMicronutrientsCarbon (C)Nitrogen (N)MIron (Fe)iHydrogen (H)Phosphorous (P)MManganese (Mn) iOxygen (O)Potassium (K)MBoron (B)i Calcium (Ca)iMolybdenum (Mo)M Magnesium (Mg)MCopper (Cu)i Sulfur (S)iZinc (Zn)i Chlorine (Cl)M
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.