The understanding of plant nutritional requirements is of importance to aquaponic growers because it provides insight on the developmental needs of the plants as they grow. This information also equips the grower with the knowledge to understand the nutrient's role in plants and helps identify the appearance and function of a healthy plant .

Of the known 92 naturally occurring elements approximately 60 have been found in plant tissue (1). Plants are non discriminatory in terms of nutrients uptake because they lack the necessary mechanism to filter out non-essential elements. Instead, plants take up the majority, if not all, of the dissolved nutrients found in soil and water solution, which is a major reason they are often used as filters in wastewater facilities (2).

What Determines If A Nutrient Is Essential?

In 1939 scientists Arnon and Stout developed 3 criteria for determining the essentiality of a nutrient in plant development and they are:

  1. A deficiency of it makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle.
  2. Such deficiency is specific to the element in question, and can be prevented or corrected only by supplying this element
  3. The element is directly involved in the nutrition of the plant as opposed to making a nutrient more available (3)

Plant Tissue Analysis

Water makes up approximately 80-95% of the plant material. The amount of water present in plant tissue depends on how turgid the leaf is during the time of sampling. turgidity is water pressure against the cell wall that swells the plant and makes it stand upright and appear stiff.

The plant tissue analysis is conducted by taking samples of the plant tissue and drying them in an oven at temperature of 70°C (158°F) for a duration of 24-48 hours. The dry weight consists only of 10-20% of the initial weight being that the majority of the plant is made up of water that evaporates from the cell during the drying process.

For example, If the initial weight of the leaf tissue is 10 g and 16% of that weight is left after the drying process, then 1.6 g is the dry weight.

10 x 0.16% = 1.6 g of dry weight

Of the 16% (1.6 g) of the remaining dry plant material over 90% of that is made up of Carbon, Hydrogen, and Oxygen. These three elements also make up more than 90% of all living organisms ranging from bacteria to Homo sapiens.

1.6 x .90 = 1.44 g (carbon, hydrogen, oxygen)

In our example we now have 1.44 g (90%) of 1.6 g (dry weight) accounted for, which leaves just 10% remaining plant material consisting of the 13 other essential elements.

Macronutrients And Micronutrients In Plant Tissue

There are a total of 16 elements that have been determined thus far to be required for a plant to complete its lifecycle. These nutrients are arbitrarily divided into 2 categories - macronutrients and micronutrients.

Below you will see a list of each nutrients and the source from which it is obtained by plants in an aquaponics system.

Macronutrients are categorized as those nutrients that are required in relatively large amounts. The Macronutrients are:

Element and its Source

Carbon (C) Carbon Dioxide

Hydrogen (H) Water

Oxygen (O) Water

Nitrogen (N) Fish Feed

Phosphorus (P) Fish Feed

Potassium (K) pH Supplementation

Calcium (Ca) pH Supplementation - source water

Sulfur (S) Fish Feed

Magnesium (Mg) Fish Feed - source water

Micronutrients are categorized as those nutrients that are required in relatively small amounts. The Macronutrients are:

Element and its Source

Iron (Fe) Chelated FeDTPA

Chlorine (Cl) Fish Feed

Manganese (Mn) Fish Feed

Boron (B) Fish Feed

Zinc (Zn) Fish Feed

Copper (Cu) Fish Feed

Molybdenum (Mo) Fish Feed

Conclusion

16 essential nutrients are all that are required to develop one of the most important forms of life on the planet. The aquaponic grower should become familiar with these elements and the understanding of how they impact the plant in various ways. In a properly balanced aquaponics system the vast majority of these nutrients will be provided by fish feed and water alone. The exception to this is Iron, calcium, and potassium which are supplemented in a fertilizer forms to make up for what the system and fish feed naturally lack.

References

  1. the essentiality of certain elements in minute quantity for plants with special reference to copper. D. I. ARNON AND P. R. STOUT
  2. Mineral nutrients of higher plants - Petra marschner
  3. Nutrients plants require for growth - Robert Mahler